Antenna wire length

[Capt'n Jack]

How long can the wire be that connects the transmitter to the antenna?
I'd like to position the antenna out front in a tree or on the mailbox post next to the road if possible.

[tmhoward]

How long can the wire be that connects the transmitter to the antenna?
I'd like to position the antenna out front in a tree or on the mailbox post next to the road if possible.

Depends on the power output. typical cable has a dB loss per meter. For instance LMR-200 has a drop of about .5 dB per meter so for every 2 meters of cable you have a full dB drop and at say 6 meters you have a 3dB drop. This would mean if you had a 1 Watt transmitter using good LMR-200 cable of say 18 feet you would radiate 0.5 Watt at the antenna.

Short answer is you need more information but using LMR-200 you don't want to go very far.

[jderba]

You would be better off putting the transmitter outside and running a long audio cable to it from the computer. I have a 50 foot audio cable running from my computer to my transmitter and it works well for me

Jack

[dnesci]

You might want to run some tests. Depending on the power you are pushing, it may not be neccessary to put it high in a tree. I have mine in the garage at the back of the house only 4 feet up the wall and it get 1/2 mile range with a .2M transmitter.

If this works for you, you can save the trouble of long wire runs.

Don

[Aurbo99]

http://www.csgnetwork.com/antennagenericfreqlencalc.html

Use the link above and type in the frequency you plan on using to transmit on.

The easiest is to take the 1/4 wave length.

example.
Freq = 99.9
Quarter wavelength = 28.10810810810811 inches or approximately 28 and 7/64 inches (28 and 1/4" is fine)

so my
Belkin antenna would be 28 1/4" long

a Dipole antenna would be 28 1/4" long ON EACH SIDE or 56 1/2" total


Now for the original question of how long can the wire from transmitter to the antenna, I am using over 80feet of RG58 snaked out my garage, around the back, up the wall and over to my antenna on my roof. (Gotta love Satellite dish mounting brakets and poles, a 1 1/2" black abs pipe with a little heat slips right over a "Dish" brand mounting pole.)

As long as you use the proper shielded cable type and BNC connectors, I doubt you have trouble with loss of signal.

[RPM]

Depends on the power output. typical cable has a dB loss per meter. For instance LMR-200 has a drop of about .5 dB per meter so for every 2 meters of cable you have a full dB drop and at say 6 meters you have a 3dB drop. This would mean if you had a 1 Watt transmitter using good LMR-200 cable of say 18 feet you would radiate 0.5 Watt at the antenna.

Short answer is you need more information but using LMR-200 you don't want to go very far.

Actually, LMR-200 isn't that bad on loss.
In the FM band it will be around 100Mhz and the loss of LMR200 at 100Mhz would be about 3.242db/100ft or 10.637/100mtrs. That works out to 0.10637db loss per meter, not .5db.

You can calculate this on the Times Microwave site at
http://www.timesmicrowave.com/cgi-bin/calculate.pl

Either way, the shorter your cable to your antenna, the better.

[scottmcm]

Is anyone using RG6 to go from transmitter to dipole? I have tons of it laying around from redoing the cable TV in the the house and using it for this would certainly help justify for the wife why I had to buy such a large spool of it. ;)

I am using the Vast V-FM01 transmitter so I am wondering if it will be OK with 75ohms?

Thanks,
Scott

[RPM]

Is anyone using RG6 to go from transmitter to dipole? I have tons of it laying around from redoing the cable TV in the the house and using it for this would certainly help justify for the wife why I had to buy such a large spool of it. ;)

I am using the Vast V-FM01 transmitter so I am wondering if it will be OK with 75ohms?

Thanks,
Scott

Scott,

The spec sheet for the V-FM01 shows an output impedance of 50 ohms, so a 75 ohm cable would not be the best choice.

[lightman]

I was under the impression that the natural impedance of a dipole antenna was just under 75 ohms and that a coax with equivalent impedance was OK.

I too saw the output impedance for the V-FM01 spec'ed at 50 ohms and was curious on the best cable to use since the dipole was closer to 75 ohms.

That said, I did some tests with simple single wire (~30 in) and then a tuned dipole using BNC connectors and 75 ohm coax. Didn't see any difference in range with either method(drove around in car listening).

Could it be the loss in the 75 ohm cable? Hmmmm.....

[RPM]

I was under the impression that the natural impedance of a dipole antenna was just under 75 ohms and that a coax with equivalent impedance was OK.

I too saw the output impedance for the V-FM01 spec'ed at 50 ohms and was curious on the best cable to use since the dipole was closer to 75 ohms.

That said, I did some tests with simple single wire (~30 in) and then a tuned dipole using BNC connectors and 75 ohm coax. Didn't see any difference in range with either method(drove around in car listening).

Could it be the loss in the 75 ohm cable? Hmmmm.....

Generally speaking, a dipole antenna is matched to the impedance of the feedline (coax) and most transmitters use an output impedance of 50 ohms.

For such a low power transmitter I would say if it works for you then great, but with a high power transmitter this could cause damage to the output section. Matching your antenna and coax to the transmitter impedance will extend your range though.

Robert

[lightman]

Generally speaking, a dipole antenna is matched to the impedance of the feedline (coax) and most transmitters use an output impedance of 50 ohms.

For such a low power transmitter I would say if it works for you then great, but with a high power transmitter this could cause damage to the output section. Matching your antenna and coax to the transmitter impedance will extend your range though.

Robert

I can appreciate the need to match all components as well as cable. Can you comment on the natural impedance of a dipole antenna?

For example, in the range of FM transmissions, would you say that the natural impedance of a dipole antenna is closer to 50 ohms than 75 ohms? I had read somewhere that they were closer to 75 ohms, thus my decision to use that same coax. Per your comments, I would conclude they are closer to 50 ohms.

Like you say, for the wattage we are talking about, it really won't hurt the output stage. Just want to get it right.

Thanks for your help.

[RPM]

I can appreciate the need to match all components as well as cable. Can you comment on the natural impedance of a dipole antenna?

For example, in the range of FM transmissions, would you say that the natural impedance of a dipole antenna is closer to 50 ohms than 75 ohms? I had read somewhere that they were closer to 75 ohms, thus my decision to use that same coax. Per your comments, I would conclude they are closer to 50 ohms.

Like you say, for the wattage we are talking about, it really won't hurt the output stage. Just want to get it right.

Thanks for your help.

The theoretical radiation resistance of a dipole is about 73 ohms. Here's a Wiki page that explains some of this http://en.wikipedia.org/wiki/Dipole_antenna

The correct way to setup a dipole would be to use 50 ohm cable to match the impedance of the transmitter, and then use a matching network such as a balun to match the dipole to the feed coax.
For such a low power transmitter I wouldn't worry about it though. My thought is if you can get enough range for someone sitting in a car outside your house to have good signal and maybe a 100 feet or so either way would be more than enough signal for blinky flashy purposes.

Robert

[rice66]

The free space (in space) impedance of a simple dipole is approximately 300 ohms.
We are dealing with alternating currents (at rf frequencies) so 300 ohms is a reactive load, not a resistive load. In rf freqs especially in higher freqs (88 to 108) mhz the losses are harder to tune out. But practically speaking, use a 50 ohm coax and a balun at the dipole. Iffn you feed the antenna close to the transmitter, a short run of coax(anytype) doesnt make enough difference to make it an engineering defeat.

There fore if you feed a pair of dipole elements with out a matching transformer you loose some of the rf energy.

BTW the three hundred (300) ohm is for a folded diploe, a simple dipole is approx 70 ohms input impedance.

Hope this helps
Rice66

[LabRat]

So my take on this (to get to the "layman's terms") is that I want to run 50ohm Coax to the antenna. This cable should have three loops (of about 6 inches diameter ) just before it attaches to the antenna.

Is this correct? (Also.. where does one typically purchase 50 ohm cable... are there markings I should be looking for?)

Thx.

[g2ktcf]

Well,

I am building the Pira xmitter....it list 50-75 Ohms as the impedance...so now I am REALLY CONFUSED.


Chris

[rice66]

The flexiability of broadband tank circuits is sometimes built into the "final stage of a transmitter, like the one you just mentioned, in that case, I would go ahead and use 75 ohm coax, either put loops at the dipole end or buy a snap on ferrite balun . , these ferrite baluns prevent radiation of the shield portion of the coax. This can travel back towards the transmitter and you loose power. that could have been raidiated. Bottom line use RG-6 or RG59 both 73 ohm coax , use the loops or purchase the ferrite snap on bead. BTW this bead can be taken from an old discarded computer monitor data lead, its the cylinderical molded device near the input plug, I have seen them on some usb cables also.
Go here and for a little over 5 bucks you can get a "RF Choke" snapon ferrite bead:

http://www.thefind.com/instruments/info-ferrite-snap

Rf cable can be obtained from radio shack, sometimes the quality of the shielding is in question. Surplus cable from tv cable companies is a good source of RG-6 (73 ohm)
good rfing
Rice66

[lightman]

The flexiability of broadband tank circuits is sometimes built into the "final stage of a transmitter, like the one you just mentioned, in that case, I would go ahead and use 75 ohm coax, either put loops at the dipole end or buy a snap on ferrite balun . , these ferrite baluns prevent radiation of the shield portion of the coax. This can travel back towards the transmitter and you loose power. that could have been raidiated. Bottom line use RG-6 or RG59 both 73 ohm coax , use the loops or purchase the ferrite snap on bead. BTW this bead can be taken from an old discarded computer monitor data lead, its the cylinderical molded device near the input plug, I have seen them on some usb cables also.
Go here and for a little over 5 bucks you can get a "RF Choke" snapon ferrite bead:

http://www.thefind.com/instruments/info-ferrite-snap

Rf cable can be obtained from radio shack, sometimes the quality of the shielding is in question. Surplus cable from tv cable companies is a good source of RG-6 (73 ohm)
good rfing
Rice66

So for clarification, does a balun create the matching impedance (reactive) for either method i.e., 1) the simple snap on ferrite bead or 2) loop of coax (3 to 4 turns 6 in diameter)?

It seems that the simple ferrite bead would only stop stray radiation back up the cable and not affect the impedance.

Thanks for your patience in these questions. I truly am interested in the theory. Nice to have it explained by one of our own.

[lightman]

I found this link on constructing simple baluns using 75 ohm coax cable....

http://www.ham-radio.com/k6sti/balun.htm

The 3 turns equates to a 3 in Diameter loop using RG59 and results in unwanted pickup at least 19 dB below the antenna signal. Do you recomend this size loop or a larger one?

Here is another DIY post that references 6 in diameter loop....
http://doityourselfchristmas.com/forums/showpost.php?p=61261&postcount=59

Question: What is the electrical difference between a 3 in diameter and a 6 in diameter loop balun? For our applications, which is best?

[rice66]

You can use both, but for practical purposes, expierment, wind the "rf Choke" and measure your distances by driving around in your car, if you want more range, try disassembling a 300 ohm to 75 ohm tv adapter and use that in conjunction with the dipole.
I still havent had time to take a pic of the ground plane antenna I use. I apologize for the delay in answering your questions. BTW , nice find the article definately is useful.
For clarification:
1. The 3 turns of coax at the antenna end is a "RF CHOKE" and as the article states, its to prevent rf current to flow on the shield of the coax, causing losses.
2. A balun wound as a 4:1 balun changes the impedance at the feed point. 50 coax to 200 0hm input......75 ohm coax to 300 ohm feed point.

I hope this helps.
Regards
Rice66

[lightman]

You can use both, but for practical purposes, expierment, wind the "rf Choke" and measure your distances by driving around in your car, if you want more range, try disassembling a 300 ohm to 75 ohm tv adapter and use that in conjunction with the dipole.
I still havent had time to take a pic of the ground plane antenna I use. I apologize for the delay in answering your questions. BTW , nice find the article definately is useful.
For clarification:
1. The 3 turns of coax at the antenna end is a "RF CHOKE" and as the article states, its to prevent rf current to flow on the shield of the coax, causing losses.
2. A balun wound as a 4:1 balun changes the impedance at the feed point. 50 coax to 200 0hm input......75 ohm coax to 300 ohm feed point.

I hope this helps.
Regards
Rice66

Very helpful. That's what I thought. The true BALUN (BALance-UNbalance) converts the impedance using a transformer arrangement and the coiled coax is an RF choke preventing extraneous rf current flow up the braid.

I will test the tighter coil (~3in diameter) tonight with a drive around the hood. One other thing I need to do is get my dipole off the wall and into free air. Will probably do something like Kevin Cook's PVC dipole.

I will look for an old 300 ohm to 75 ohm balun.... I am sure I have one around here somewhere.

Not to worry about the pictures. When it gets done it gets done.

[lightman]

Very helpful. That's what I thought. The true BALUN (BALance-UNbalance) converts the impedance using a transformer arrangement and the coiled coax is an RF choke preventing extraneous rf current flow up the braid.

I will test the tighter coil (~3in diameter) tonight with a drive around the hood. One other thing I need to do is get my dipole off the wall and into free air. Will probably do something like Kevin Cook's PVC dipole.

I will look for an old 300 ohm to 75 ohm balun.... I am sure I have one around here somewhere.

Not to worry about the pictures. When it gets done it gets done.

Ran two tests tonight ....one with 3 in diameter coax coil and one with 6 in diameter coax coil (rf Choke). Results showed the 6 in coil worked better by almost 2 to 1.

The 3 in coil produced more hiss and break up at around .1 miles while the 6 in coil held up to about 0.2 Mile. Going north I hit 0.3 mile (looks like asymetric lobes).

Set up was V-FM01 transmitter with iPOD audio feed (3/4 volume setting). Output of transmitter (50 ohm impedance) connected through BNC to 6 ft of RG59 (75 ohm) coax to BNC extender to another 6 ft length of RG59 that had 3 turns (6 in diameter coil). Coil was connected to dipole BNC. Freq = 94.1 MHz and 1/4 L = 29 7/8 in. Used 22 AWG stranded wire (with insulation) for both signal and ground antenna wires that were taped to wall (Dry Wall). Yes... I have a mismatch at the xmitter output to RG59 cable, but it was same for both tests.

Will build stand alone dipole this weekend and place outside to increase range.

[g2ktcf]

so the 3.5 turns at 6" diameter seemed to be the best.

I am curious though....the dipole wire should be bare..not insulated. I wonder how much of a difference this would make?

[rice66]

Practially speaking, the insulation on the wire for the dipole isnt a factor, plastic appears opaque to rf, especially at vhf frequencies. The best rule of thumb is to suspend the dipole on an rigid support ( ie: plastic pvc pipe, fiberglass , waterproofed wood. The antenna portion connected to the center conductor should be suspended as far away from house structures.
Rule of thumb, calculate 1/4 wavelength at your operating frequency and suspend the antenna at least 1/4 away from any structure, suspend it in multiples of 1/4 wavelengths
ie: 1/4, then 1/2 ;then 3/4 then 1 , then 1 1/4 wavelengths.
Suspend the antenna in a verticle manner. This establishes a vertical polarization of the rf energy and presents maximum exposure to vertical antennas , such as those of car antennas.

Happy rfing
Rice66

[g2ktcf]

Practially speaking, the insulation on the wire for the dipole isnt a factor, plastic appears opaque to rf, especially at vhf frequencies. The best rule of thumb is to suspend the dipole on an rigid support ( ie: plastic pvc pipe, fiberglass , waterproofed wood. The antenna portion connected to the center conductor should be suspended as far away from house structures.
Rule of thumb, calculate 1/4 wavelength at your operating frequency and suspend the antenna at least 1/4 away from any structure, suspend it in multiples of 1/4 wavelengths
ie: 1/4, then 1/2 ;then 3/4 then 1 , then 1 1/4 wavelengths.
Suspend the antenna in a verticle manner. This establishes a vertical polarization of the rf energy and presents maximum exposure to vertical antennas , such as those of car antennas.

Happy rfing
Rice66

Did not know about the offset for the shield side....darn, that makes mounting a bit more difficult as I was planning on bolting this to my chimney...well...back to the drawing board.

[IdunBenhad]

Hi:
I've been off for awhile--Been traveling. Right now I'm in Gettysburg, PA.

Anyway, almost everything the above posts have said is true, but it seems to me that everyone is busy worrying about range when it is not necessary. Ask yourself this question: "Why must my signal go beyond my lot or even my block?"


The answer is, of course, so long as the car can reliably receive my signal without noise, in front of my display, then I don't need to go great lengths to get the antenna high in the air and the signal go all over the neighborhood.

If that is the case, then coax loss, transmitter power output and antenna type or height, becomes irrelevant. All you are looking for is to get a good signal into your visitors car. You don't need to broadcast to the world.

Just use good coax and connectors, match impedance, and everything should work out fine. It is not necessary to go to the higher expense of LMR200. Use RG58x coax (note the "x" on the RG58, it indicates a lower loss coax) and you're good to go.

Just a thought.

Idun

[g2ktcf]

Hi:
I've been off for awhile--Been traveling. Right now I'm in Gettysburg, PA.

Anyway, almost everything the above posts have said is true, but it seems to me that everyone is busy worrying about range when it is not necessary. Ask yourself this question: "Why must my signal go beyond my lot or even my block?"


The answer is, of course, so long as the car can reliably receive my signal without noise, in front of my display, then I don't need to go great lengths to get the antenna high in the air and the signal go all over the neighborhood.

If that is the case, then coax loss, transmitter power output and antenna type or height, becomes irrelevant. All you are looking for is to get a good signal into your visitors car. You don't need to broadcast to the world.

Just use good coax and connectors, match impedance, and everything should work out fine. It is not necessary to go to the higher expense of LMR200. Use RG58x coax (note the "x" on the RG58, it indicates a lower loss coax) and you're good to go.

Just a thought.

Idun


Well, one reason that I have been watching this is to see if I can get my mod'ed Belkin to work as a backup. My signal from it is horrible.

[lightman]

Thanks for the informative explanation Rice66. Nice to know about the standoff as I construct my dipole this weekend. I didn't think the plastic insulation was an issue for these frequencies.

Being an engineer/scientist I like putting theory into practice, test the limits and then adjust for practical application. Besides....it is fun!

In the end, I agree with Idunbenhad, I only need to cover my street with crisp, clear, no hiss audio. I also do audio engineering on the side, so clean audio is paramount for me.

Have fun....

Lightman