Funny Dipole Question

[scottmcm]

OK, I've read through all the forum posts about dipole antennas and the wikis. I think I get the basics of how they work and what they do. I have a question though....and please don't laugh....

Since the dipole assembly is basically a coax wire running some length which then becomes two strands of copper of a fixed length the whole unit is really just two long wires. In that case, how do the wires know when to stop being wires and start being antennas? How do they know that only the last 2 feet or so are all we need to radiate from? Lastly, is the measurement for the copper "poles" from the point where it splits off from the connector or from the point where the wire turns the corner in the "T"?

Thanks ya'll!

[Santa Don]

Scott, basically, the conductor inside the braid of a coax is shielded and prevented from radiating a signal. They sort of cancel each other out.

[scottmcm]

Makes sense. Thanks Don. I knew it was something painfully obvious but I just wasn't seeing it. One of the wikis I read talked about signal loss from the coax so I let my imagination run with it.

[lightman]

I recently acquired a VAST V-FM01 board and tried two different antenna configurations, but saw no real discernable difference. Both were clear for about 2 block, so it meets my needs. Just curious why I didn't see better performance with dipole.

First one was a quick 30 in stranded wire soldered straight to the board (no ground wire). It was sitting on benchtop, upstairs, wire horizontal running on 94.1 MHz. It got about 1/10 mile radius coverage.

Second was a dipole tuned to 94.1 MHz (~29.84 in per side - 1/4 wavelenght) fed through 6 ft of 75 ohm coax with BNC connectors on both ends. Antenna was mounted upstairs on a wall in a vertical configuration with ground wire down. It also covered about 1/10 mile, same as single wire. Dipole wires were taped to a wall with BNC connector in the middle.

I thought I would have gotten much better performance with dipole. Am I doing something wrong?

[ukewarrior]

Did you put three or so turns in the coax right before it attaches to the dipole to create a balun?


I recently acquired a VAST V-FM01 board and tried two different antenna configurations, but saw no real discernable difference. Both were clear for about 2 block, so it meets my needs. Just curious why I didn't see better performance with dipole.

First one was a quick 30 in stranded wire soldered straight to the board (no ground wire). It was sitting on benchtop, upstairs, wire horizontal running on 94.1 MHz. It got about 1/10 mile radius coverage.

Second was a dipole tuned to 94.1 MHz (~29.84 in per side - 1/4 wavelenght) fed through 6 ft of 75 ohm coax with BNC connectors on both ends. Antenna was mounted upstairs on a wall in a vertical configuration with ground wire down. It also covered about 1/10 mile, same as single wire. Dipole wires were taped to a wall with BNC connector in the middle.

I thought I would have gotten much better performance with dipole. Am I doing something wrong?

[reidfo]

I recently acquired a VAST V-FM01 board and tried two different antenna configurations, but saw no real discernable difference. Both were clear for about 2 block, so it meets my needs. Just curious why I didn't see better performance with dipole.

First one was a quick 30 in stranded wire soldered straight to the board (no ground wire). It was sitting on benchtop, upstairs, wire horizontal running on 94.1 MHz. It got about 1/10 mile radius coverage.

Second was a dipole tuned to 94.1 MHz (~29.84 in per side - 1/4 wavelenght) fed through 6 ft of 75 ohm coax with BNC connectors on both ends. Antenna was mounted upstairs on a wall in a vertical configuration with ground wire down. It also covered about 1/10 mile, same as single wire. Dipole wires were taped to a wall with BNC connector in the middle.

I thought I would have gotten much better performance with dipole. Am I doing something wrong?

I'm getting similar results with my Ramsey FM-25B. Using the built-in telescoping antenna I can get about 2 blocks from the house (I think about 1/10 mile). I built a dipole antenna tuned to 91.7MHz that gets the exact same range. The antenna is about 4 feet of 75ohm coax soldered to stranded copper wire for signal and ground. The antenna is placed vertically with ground down. I put a large ferrite core close to where the coax meets the antenna, with two turns of coax around the ferrite (all I could get through it).

I thought my dipole would get much better range than the simple telescoping antenna.

[lightman]

Did you put three or so turns in the coax right before it attaches to the dipole to create a balun?

Ahhhh... I read about the balun in earlier posts and forgot to add. Thanks for the heads up.

[lightman]

Ahhhh... I read about the balun in earlier posts and forgot to add. Thanks for the heads up.

Success!

Added ~6 in loop of coax just before tuned dipole antenna (94.1 MHz)- about 3 to 4 turns (no ferrite core - didn't have one). Had to add another 6 ft of coax (w/ BNC connectors) to reach transmitter. Did range test (drove around in truck) and saw about 100% improvement in range! That is I appear to be getting ~ 0.2 miles radius of coverage, up from prior 0.1 mile radius.

Note: I did do the original test during the day and this new test at night. Heard that coverage (range) will improve at night... so that may be part of the improvement.

Again, this is more range than I need for my front yard and street. Just want to see what it can do.

Thanks for you help...

Lightman

[rice66]

Antennas is my speciality hobby (amateur radio).Basically, when you come from a transmitter the final tank area(last tuned parts) is normally tunned for 50 ohms (impedance).
If you attach a 50 impedance coax, the match is perfect and the power will be transmittecd into the coax. All goes well until the end of the coax. If you have an antenna that is 50 ohms impedance the power will be transferred to the (load) antenna. If there is a mismatch anywehre the power will be reflected back to the transmitters output tank area. This causes swr (standing wave reflection.) the power is pushed back on itself causing loss in the form of heat and unwanted radiated waves.
Puting a few turns of coax in the form of a balun helps because this makes a tank circuit at the end of the coax which is (hopefully) tunned to the coax's impedance. That in turn transfers maximum power to the antenna (load)

I noticed you were using 75 ohm coax to feed a dipole, first mismatch is the 50 ohm output to 75 ohm coax. the next mismatch is the 75 ohm coax to a 300 ohm dipole.

Also it is important to use a coax that has great shielding, such as RG214 or 50 ohm helix coax tubing.
Sincde most of us dont have that good of supplies, using a short (very short) run of RG58 will be ok. Make the balun as close to the antenna as possible. I took apart one of the old ntenna adapters for a TV. The adapter had a balun inside, making it a 4 to 1 balun. I put the 50 ohmcoax to this adapter and it made a better match, not perfect, but increased the amount of radiated rf higher.

I have a communications reciever with a S meter. I tunned the radio to my clear channel. By watching the S meter I was able to make a noticable difference with the aforementioned improvements.

My antenna finally became a ground plane antenna with 4 radials and 1 vertical element. The feed point was made with a salvaged cb antenna mount. the ground plane driven element was fed at the base with no balun, because the helix coax was 50ohm impedance and the vertical antenna is approx 50 ohms nominal impedance. Range was over the legal limit (approx 2 miles line of sight).
Transmitter was the tunecast II modified to drive a ramsey 1 watt unit, the unit was throttled down a bit to stay close to legal limits.
Hope the information wasnt too technical or too lenghtly.
Happy holiday lighting projects
Rice66

[rice66]

My last post had a mistatement in it. The free space imped ance of a (folded)_ dipole is 300 ohms nominlly. The actual impedance at the input of a dipole is approx 75 ohms

[reidfo]

I noticed you were using 75 ohm coax to feed a dipole, first mismatch is the 50 ohm output to 75 ohm coax. the next mismatch is the 75 ohm coax to a 300 ohm dipole.


So should the dipole also be 50ohm? What type of wire should be used for that?



Hope the information wasnt too technical or too lenghtly.
Happy holiday lighting projects
Rice66

Not at all. Thanks for the help!

[rice66]

Yes there is a 50 ohm input antenna. its called a vertical ground plane antenna.

http://www.hamuniverse.com/2metergp.html

the antenna mentioned in this article is for 144mhz range, just make the radials and verticaL radiator longer for 88 thru 108 mhz. use the formula presented there. I mounted my so-239 chassis connector on a 1/8" 6inch square of scrap aluminum plate. The radials I used were brazing rods, cut to length, the radiator was a stainless steel discarded antenna . If you use a pl239 connector, simply solder a brazing rod thru the center connector and fill the area around the plug with an epoxy substance. A good 5 min two part epoxy works well.
Good rfing
Rice66

[rice66]

If yopu have a friend with a reciever with an "S" meter, you can adjust the radiators angle and the antenna hieght to obtain the best "50" ohm reactance. Antenna hieght in multiples of the wavelength, plus the angle of the radiators create a near perfect match. This would be the higest S-1 meter reading. Also when you come to the antenna try to route the coax away from the antenna in a 90 deg angle as far as practical, before you make the run to the transmitter. If your coax is a short run, put a rf choke on it and go with it. You get what you can in that case.


Practically speaking, bend the ground plane radials to a 45 deg angle from the vertical and put the antenna away from trees ,metal and the like and as high as you can. , make the rf choke as described or buy a ferrite snap-on choke and place near the antenna . 2 or 3 inches from the antenna input. My freq here is 105.6 fm and I cut my radials and vertical driven element to approx 31".
Good rfing
Rice66

When I get home I will post a pic of the home made antenna and the heliax cable.

[sandy]

My last post had a mistatement in it. The free space imped ance of a (folded)_ dipole is 300 ohms nominlly. The actual impedance at the input of a dipole is approx 75 ohms

Now I don't have to correct you.
Thanks for helping.

I made a wire J-pole for the 88.1 MHz frequency I'm using.
Put it inside a pvc pipe and stuck it out the top of the megatree.
A bit too much wire loss, but the coverage due to the height is great.

W1CGS

[lightman]

If yopu have a friend with a reciever with an "S" meter, you can adjust the radiators angle and the antenna hieght to obtain the best "50" ohm reactance. Antenna hieght in multiples of the wavelength, plus the angle of the radiators create a near perfect match. This would be the higest S-1 meter reading. Also when you come to the antenna try to route the coax away from the antenna in a 90 deg angle as far as practical, before you make the run to the transmitter. If your coax is a short run, put a rf choke on it and go with it. You get what you can in that case.


Practically speaking, bend the ground plane radials to a 45 deg angle from the vertical and put the antenna away from trees ,metal and the like and as high as you can. , make the rf choke as described or buy a ferrite snap-on choke and place near the antenna . 2 or 3 inches from the antenna input. My freq here is 105.6 fm and I cut my radials and vertical driven element to approx 31".
Good rfing
Rice66

When I get home I will post a pic of the home made antenna and the heliax cable.

Very informative! Thanks. Can't wait to see the picture.

I will convert to 50 Ohm Coax and continue to use the Balun. Saw the update on Dipole impedance of 75ohm (as I expected). Your post makes it clear that the mismatch is best handled at the antenna and not at the transmitter.

Good Post...thanks again.

[lboucher]

HI All

Thought i would chime in since i just tested my setup this weekend.
I played with this setup last year a bit and got ~300 yards.

This time, i have a 50mw transmitter from china.
I am using RG59 (But now that i look at that i realize it has an impedance of 75 ohms, i thought it was 50 ohms.)
So from transmitter -> bnc to f female adapter -> ~6' of RG59 -> impedance adaptor -> radio shack dipole antenna.

Then i got the antenna outside up high, bottom of antenna is ~10' off the ground.

Now i get a range of ~ 1 mile solid and the farthest i saw was ~2 miles.
Would probably go farther if i could find a higher area away from building and power lines.

(Gotta go home and check that cable and get RG58 if the impedance is wrong.)

[lightman]

HI All

Thought i would chime in since i just tested my setup this weekend.
I played with this setup last year a bit and got ~300 yards.

This time, i have a 50mw transmitter from china.
I am using RG59 (But now that i look at that i realize it has an impedance of 75 ohms, i thought it was 50 ohms.)
So from transmitter -> bnc to f female adapter -> ~6' of RG59 -> impedance adaptor -> radio shack dipole antenna.

Then i got the antenna outside up high, bottom of antenna is ~10' off the ground.

Now i get a range of ~ 1 mile solid and the farthest i saw was ~2 miles.
Would probably go farther if i could find a higher area away from building and power lines.

(Gotta go home and check that cable and get RG58 if the impedance is wrong.)

Wow! That is quite the distance. For comparision, my V-FM01 is rated at 15 mW and the antenna is upstairs (~15 to 20 ft up) inside the house on taped to a wall in my office/lab. As reported, it gets a range of about 0.2 mile.

I am thinking about moving the antenna outside to test for maximmum range, however, I am concerned about violating FCC rules... so will probably leave indoors to reduce range and protect from elements.

It's funny ... the engineer is me wants to tune for absolute best performance, but then in real use, I will attenuate down to keep within the rules.

Gotta love this hobby!

[rice66]

I second the tune for maximum performance. I put up an antenna (and you gotta love the dish networks brackets, left in place) using that and 1/4 wave legnth above the roof surface, got me 2 miles coverage line of sight, and static and dead spots in a crowded neighbor hood.(I am using a modified tune cast II and a ramsey 1 watt broadband linear)By using a degernative feedback circuit, I reduced the output enough to give me 3 blocks in all directions. There are some dead spots created by brick homes and tall trees, but by the time you see my display, the music is solid.
Happy rfing
Rice66

[dirknerkle]

I second the tune for maximum performance. I put up an antenna (and you gotta love the dish networks brackets, left in place)...

Wonder if you could use the dish and send the signal directly up to the satellites... :rolleyes: and get some REAL coverage! Just think -- you could be heard all 'round the world!!! (uh.... on second thought... the feds probably wouldn't like that too well...)

[lboucher]

So now i need to ask, anybody ever got in trouble?
Also has anyone broadcasted over a distance of several miles all season without any FCC cease and desist orders?

[lightman]

Wonder if you could use the dish and send the signal directly up to the satellites... :rolleyes: and get some REAL coverage! Just think -- you could be heard all 'round the world!!! (uh.... on second thought... the feds probably wouldn't like that too well...)

Now that would be cool.... We all use the same sequence and have the music piped down to the entire world. Blinky flashy in unison.... in every country.

That would have to get us into the Guiness Book of Records!

[g2ktcf]

So now i need to ask, anybody ever got in trouble?
Also has anyone broadcasted over a distance of several miles all season without any FCC cease and desist orders?

I have not heard of anyone getting busted for this. However, wvengineer had a buddy with the FCC come out to check out his transmitter. It seems that if someone complains, the white trucks show up..spend several hours setting up monitoring equipment...then listen for you pirate broadcast. Since most of us only transmit a few hours every night, it is really hard for all this to come together to actually get into trouble.

In other words...

1. Someone has to to complain to the FCC
2. FCC has to have time to investigate
3. You have to be broadcasting WHILE they are investigating.

Even if you are running high wattage its hard for these three things to happen all at the same time.

Cheers,
Chris

[rice66]

yes to the FCC monitoring. Although it be illeagle, and I wouldnt propose anyone run several watts to get a clear signal 1 mile or less away, The FCC will act decisively if you interfere with commercial licensed FM stations.
You must do your homework, search for a clear channel in your area and make sure that your transmitter doesnt broadcast trash on near by active channels. You must also identify your station via some identifiable means once at least every 10 minutes.
IE: The is the smiths christmas display radio , broadcasting 5 pm to 10 pm daily. Broadcasting on a frequency of 105.6 mhz etc

Happy rfing
Rice_66

[IdunBenhad]

Hi:
Want to make an easy dipole? Remove the outside covering from your coax for 1/2 the total length of your dipole. Very carefully remove the center conductor with its' insulation out of the braid where the outside covering that you removed stops. Use a plastic tie-wrap around the coax. Stretch the two conductors (braid is one, center conductor the other) out like a dipole and there you are. A dipole with no connectors and no soldering at the antenna end.

Idun

[lightman]

Here is an update on my transmitter set up. I purchased 12 ft of RG58 (50ohm) BNC coax and connected the V-FM01 to dipole antenna. I added a "snap on Ferrite Data Line Filter" from Radio shack instead of the 3 turn loop and slid it up close to the Dipole connection.

Performance was not good. This configuration had 0.1 Mi radius coverage as compared with > 0.2 Mi radius using the RG59 (75ohm) BNC coax with 3 turn loop.

I will have to try the RG58 with 3 turn loop tomorrow night and see if performance is any better. Thought the 50 ohm coax was better match for the system.

Not sure if the snap on ferrite filter is right one. It fits very loose over the cable.

Does it need to be tight against the outer coax insulation?

Update 20 minutes later.... I ran test with 3 turn loop (6 in dia) with RG58 (50ohm) coax and saw no improvement over the snap on ferrite filter. Still only 0.1 mi radius coverage.

Looks like RG59 (75 ohm) coax with 3 turn coil loop is best with my set up. Still curious about how tight the ferrite filter needs to be.

[g2ktcf]

Here is an update on my transmitter set up. I purchased 12 ft of RG58 (50ohm) BNC coax and connected the V-FM01 to dipole antenna. I added a "snap on Ferrite Data Line Filter" from Radio shack instead of the 3 turn loop and slid it up close to the Dipole connection.

Performance was not good. This configuration had 0.1 Mi radius coverage as compared with > 0.2 Mi radius using the RG59 (75ohm) BNC coax with 3 turn loop.

I will have to try the RG58 with 3 turn loop tomorrow night and see if performance is any better. Thought the 50 ohm coax was better match for the system.

Not sure if the snap on ferrite filter is right one. It fits very loose over the cable.

Does it need to be tight against the outer coax insulation?

Update 20 minutes later.... I ran test with 3 turn loop (6 in dia) with RG58 (50ohm) coax and saw no improvement over the snap on ferrite filter. Still only 0.1 mi radius coverage.

Looks like RG59 (75 ohm) coax with 3 turn coil loop is best with my set up. Still curious about how tight the ferrite filter needs to be.


But did you try the loops WITH the Ferrite?

[lightman]

But did you try the loops WITH the Ferrite?

No I didn't use loops within the ferrite choke. Ran single line through ferrite core.

I have seen where some ferrite chokes allow enough room to loop the cable through the the core, 1 to to 3 times. This choke unit has max diameter wire of 17/32 in which allows one loose single RG58 coax but no more. Tried to run one complete loop through (i.e., 2 coax diameters) but upon closing the ferrite halves, it appeared I would crush the cable, so stopped.

OR... did you mean use the Ferrite choke on single coax line and then add the 3 loop coil further down the cable, external to the choke? No, I didn't do that either. Didn't think one needed both.

[WireWrap]

Here is an update on my transmitter set up. I purchased 12 ft of RG58 (50ohm) BNC coax and connected the V-FM01 to dipole antenna.
...


The RG-58 you purchased may be lossier than the RG-59, possibly much lossier. For example, at 100 MHz Beldon 8240 (RG-58) is listed with a loss of 3.8db/100', where Beldon 8241 (RG-59) is only 3.4db/100'. In either case, after 100' you'll have less than half your original signal strength. 3.0db is the half-power point.

Cheaper brands of coax are just that - cheaper - and very likely more lossy than Beldon (a major cable manufacturer). So when you start off with a small signal and run it through a lossy line, you're not gonna have much to feed the antenna. Which is probably good for this hobby. You really don't need to have your signal much past the boundaries of your display, and there's less chance of incurring unwanted attention.




Thought the 50 ohm coax was better match for the system.
...

It probably IS a better match to the FM-01, however it is a worse match for the antenna, unless you use a matching transformer/balun to match the impedance of the antenna (around 73 ohms) to 50 ohms (RG-58 and nominal Tx output stage).


.

[scottmcm]

Its great how my funny question has started such a lively discussion of dipoles. So let's keep it going.

I have pile of coax with no markings. How can I tell what it is? Is there a way to measure the impedance with a simple multimeter?

[lboucher]

Hi All

Sorry i kinda got another thread going on, on the topic of impedance. Should have just joined this one.
http://doityourselfchristmas.com/forums/showthread.php?t=8284

The questions were.
Is a radio shack dipole antenna 75 ohm or 300 ohm?
Can you buy a 50 ohm to 300 ohm matching transformer, i cannot seem to find one.
What is the best way to cable this up.

Assuming 50 ohm transmitter:
Is it better to go, 50 ohm bcn cable -> BNC to f adapter -> 75 ohm to 300 ohm matching transformer -> Radio Shack dipole.

OR

is it better to go, BNC to F adapter -> 75 ohm cable -> 75 ohm to 300 ohm matching transformer -> Radio Shack dipole.

Thoughts?

[Macrosill]

My personal opinion with these low power transmitters is to eliminate the 75-300 ohm balun. Go straight from the coax to the dipole. In theory the balun is the right thing to use but in reality I find better performance from the low power transmitters with out it.

edit: see 2 posts below in reference to the use of a balun.

[dirknerkle]

My personal opinion with these low power transmitters is to eliminate the 75-300 ohm balun. Go straight from the coax to the dipole. In theory the balun is the right thing to use but in reality I find better performance from the low power transmitters with out it.

I'd agree. I mean, geez.... we're talkin' about a $40 part... and really only a distance of maybe a couple hundred yards. After that, who cares??? I mean, engineering is one thing, but overengineering is another.

[Macrosill]

I copied this from wikipedia. This may explain why I find better performance with no balun at all. A dipole has a 73 ohm impedence. A folded dipole has a 300 ohm impedence. So by putting a 75-300 ohm transformer there you are making things worse, if you are NOT using a folded dipole.


Feeder line

Ideally, a half-wave (λ/2) dipole should be fed with a balanced line matching the theoretical 73 ohm impedance of the antenna. A folded dipole uses a 300 ohm balanced feeder line.

Many people have had success in feeding a dipole directly with a coaxial cable feed rather than a ladder-line. However, coax is not symmetrical and thus not a balanced feeder. It is unbalanced, because the outer shield is connected to earth potential at the other end. [2] When a balanced antenna such as a dipole is fed with an unbalanced feeder, common mode currents can cause the coax line to radiate in addition to the antenna itself, and the radiation pattern may be asymmetrically distorted. [3] This can be remedied with the use of a balun.

[lightman]

I agree with Macrosill and dirknerkle having done quite a bit of testing, researching, and reviewing posts on this forum. I have come to an end of my transmitter journey.

My Findings: I have concluded that with a simple dipole antenna and VAST's V-FM01, that a direct RG59 (75 ohm) coax from the transmitter to the dipole with a 3 turn (6 in dia) coil in coax just before the connection to the dipole (without a "transformer" balun) is the best match for my system.

This is justified since the natural impedance of a simple dipole is ~ 73 ohms matches the 75 ohm coax. The 3 turn coil eliminates back rf, reducing radiation of the feed line but does not change impedance (this did help with range too).

The alledged mismatch from the transmitter out (advertised at 50 ohms) and the 75 ohm coax is nothing to worrying about. Some research reports that it is better to have a mismatch at the transmitter output than at the antenna connection. Readings tell me that any reflected wave can travel back to output and it is more important to dump power into the circuit (trans line and antenna) to minimize power loss in the final stage. See primer on impedance here....

http://www.jaycar.com/images_uploaded/impmatch.pdf

Status on Build: I have completed my final dipole antenna with mounting brackets (similar to Kevin Cook's design, but with a twist). Yesterday's testing showed additional improvement in range (another 0.1 mile) due to keeping vertical dipole at least 1/4 wavelength away from any structure (antenna was indoors upstairs). Final tests outside will be done this weekend. Either way, I have crisp clear, audio through my entire street (primary goal) and I am no l longer worried about burning out transmitter.

I would like to thank all the patient readers and contributers to my quest. This forum is absolutely the best for communicating ideas and transfer of info. Kudos to the forum administrators... job well done.

I will post photos of final antenna and construction once I figure out how to attach photos.

Now back to the blinky flashy part of the show....

Regards,
Lightman

[lightman]

Here is block diagram of Dipole Antenna that I built (updated to Rev 2). Also is photo of final assembly. Block diagram corrected to show Center Conductor and Outer Shield on Dipole legs.

It includes mounting piping and hardware along with access for coax pass throughs. Dipole BNC is inside elbow that allows Coax to pass through stand off arm. It not only allows a structure mounting point, it also keeps the feed perpendicular to the attenna connection, provides 1/4 wavelength stand off, and can be easily disassembled for storage.

Having trouble with attachments and images. If this works, I will post more detailed photos.

[lightman]

Here is second set of detailed construction photos. Dipole BNC is attached to a Fender Washer then inserted into the elbow for gluing (5 min epoxy).

Mount is made by drilling hoe and screwing in PVC 1 in adaptor. This wooden mount can be screwed in wall, tie wrapped to tree, mounted on satellite post, etc.

[lightman]

Last set of construction photos show cutting slit for coax coil and other pass through. Final parts are laid out for assembly.

Will photograph final antenna with coax run and post later. FYI...The Coax Coil hangs under extension arm, with slit down so that water will not collect.

[dirknerkle]

Very well done! Love the photo story, too! :D

[lightman]

I have mounted the antenna outside (see photo below) and did drive test at night. Range increased from 0.2 mile to 0.5 mile radius.

Antenna was mounted on top of back patio upstairs porch. Used bunge cords to lash to post. Set up transmitter within a few feet along with iPOD.

Other photos show the underside of the Standoff arm revealing the slots for BNC coax. You can also see the 3 turn coax loop (6 in dia) hanging from under the arm.

Very pleased with results.

Now to enclose the transmitter and weatherize the setup.

Our work is never done....."Hi Ho... Hi Ho, It's off to work we go...."

[lightman]

Very well done! Love the photo story, too! :D

Thanks .... Finally done!

[WireWrap]

I have mounted the antenna outside
...
Our work is never done....."Hi Ho... Hi Ho, It's off to work we go...."



At least you're done with that project. Thanks for taking the time to share for all the others that may see it later. You have a well-documented discussion and final result to your credit. :D :D :D

[rice66]

Very well done Lightman, fed with RG59 you should geter done!
Attached is my take on antenna, because I wanted to use 50 ohm coax from low power trans to antenna 50 impedance. Beware this is an homebrew project (he he) My brew is called a ground plane antenna. You can google this antenna and find many specifications, basically I used 1/4 wavelengths at my operating frequency, as calculated using a formula I have used successfully for 50 + years (1957 to now). I use 468/ freq in mhz. This is for a half wave calculation, I just halve the results for 1/4 wavelengths.
I used an old cb whip antenna mount, its insulated with nylon feed thru insulators thru a aluminum bracket. One end is threaded with 3/8 in common antenna screw mount, and the other end is fixed with a pl239 rf connector. I bent a scrap piece of 1/8 in aluminum plate and mounted u clamps to mount it on a abandoned dish network roof mount.
The radials have been bent nearly flat during my storage. When I finish installing the antenna I bend the radials down to a 45 degree angle from the vertical driven element. The radials are made from brazing rod and easily bend to the correct angle and are stiff enough to hold the angle . I have tried this antenna without a ground, and it works ok. I tried a ground on the metal dish network mount (12ga) ground wire rundown to my electical service entrance ground and I noticed some better coverage. Besides that if we somehow get thurnder snow storms(which we have had) the antenna would be grounded better.
Have a great RFing event
Rice66

[Macrosill]

Lightman,
You did a great job on that antenna and this tutorial. I would like to make one comment though. For better understanding in the diagram you refer to the dipole leads as "hot leg" and "ground leg". I think it would be better to refer to them as the "center conductor" and "outer shield". Some may not realize the center conductor is the "hot" leg and should be the top half. Others may simply question what is the hot leg? Just a thought.

Otherwise great job!

Would you be willing to make a new thread in the How-To section detailing the construction so others can find it easier?

[lightman]

Macrosill,

Thanks for the catch. Funny thing is, I debated whether to call it "Hot" or "Center Conductor" when I made up the diagram. I will update with an edit to correct that and the "Outer Shield".

Yes, I don't mind creating a new thread in the "How To" section. Is there a way to move these threads easily. i.e. port them without re-typing?

I can do it either way.

UPDATE - Corrections made to diagram 10/4/09, 10:22 am

[lightman]

As a postscript to my threads on transmitter and antenna, I completed the enclosure (similar to ukewarrior's solution) and have included a few photos here. Box was purchased from Wal-Mart for about $7.

I chose to mount a BNC connector right on the V-FM01 board to minimize any losses, however, that limited my mounting options inside the enclosure. I had to move the board to the right (bias it over) to allow for the chassis mount BNC connector. Positive side is that it leaves room on the left for a switching regulated 12 VDC supply if needed at a later time. Standoffs were used on the other three corners. The chassis mount BNC conflicted with the forth standoff, so I left it off.

Other connections, power and audio were brought out to the edge with pigtail wires to the circuit board.

All in all, very pleased with the result. Thanks ukewarrior for the idea.

Regards,
Lightman

[dirknerkle]

That is one of the cleanest, efficient-looking solutions I've ever seen. Really nice work!!!

[lightman]

Thanks dirknerkle, but the credit goes to ukewarrior. He inspired me to pursue those watertight enclosures readily available at Wal-Mart (sporting good section). His prior post (different thread) showed a similar configuration.

The hard plastic drills very nice. Even a forstner bit for the 1/2 in holes worked great. The see through plastic makes typical blind drill points very visible.

Don't you love the forum for sharing ideas... saves so much time.

I am now trying to see if I can fit my 8 channel triac controller into the next size up (~$8.00). Problem is the power supply wall wart is just a bit too large. I don't want to install it outside the box, since they controller will be out in the elements.

I will figure something out....

Regards,
Lightman

[dirknerkle]

I am now trying to see if I can fit my 8 channel triac controller into the next size up (~$8.00). Problem is the power supply wall wart is just a bit too large.

My wife always says that "anytime you want to make something smaller, just put it in the dryer on the "permanent press" cycle for a half hour or so..." ;)

[lightman]

LOL! Very good. I will try it and see if it works.

[ukewarrior]

Nice Work Lightman.

In all fairness, I also got my inspiration from another forum member too.

You are correct in that these forums allow a ton of great idea sharing.

Those watertight backpacking cases from WalMart are very nice for the money.

I suggest folks post their solutions to mounting these transmitters as they build them.


As a postscript to my threads on transmitter and antenna, I completed the enclosure (similar to ukewarrior's solution) and have included a few photos here. Box was purchased from Wal-Mart for about $7.

I chose to mount a BNC connector right on the V-FM01 board to minimize any losses, however, that limited my mounting options inside the enclosure. I had to move the board to the right (bias it over) to allow for the chassis mount BNC connector. Positive side is that it leaves room on the left for a switching regulated 12 VDC supply if needed at a later time. Standoffs were used on the other three corners. The chassis mount BNC conflicted with the forth standoff, so I left it off.

Other connections, power and audio were brought out to the edge with pigtail wires to the circuit board.

All in all, very pleased with the result. Thanks ukewarrior for the idea.

Regards,
Lightman

[lightman]

Sounds good ukewarrior...and thanks for your inspiration.

BTW...Good luck on your new buy of V-FM02s.

[djulien]

Here is block diagram of Dipole Antenna that I built (updated to Rev 2). Also is photo of final assembly. Block diagram corrected to show Center Conductor and Outer Shield on Dipole legs.


In the photos, I think I see some duct tape holding the loops together - are the loops more effective when held closely together like that, or was that just for convenience to keep them from spreading out? If taping them together affects their performance, then should the loops be in order (first loop on top when looking perpendicular through the open loops, last loop on the bottom, etc), or just any order? (are these just loops, or are they forming a coil?)

Also, does it matter if the loops go up vs. down?

don

[lightman]

The loops should be side by side as tight as possible, not on top of each other. The masking tape simply keeps them together. Tie wraps will also work. It is also important to use 3 coils. This effectively keeps radiation from flowing back up through the shielding that can get into other eqiupment causing noise and other issues.

I don't know if the orientation of the coil plane to the dipole antenna is important. I had it hang down for convenience.

The coil should be as close to the antenna connection as possible. Also, having the coax leave the dipole perpendicular to the legs for 1/4 wavelength helps.