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View Full Version : dipole - developing a mini-howto



xmus
11-14-2007, 09:42 AM
The first step in making a dipole is to select a frequency. In my case it was 101.3MHz. There are thousands of dipole calculators online, i Googled for one: http://www.kwarc.org/ant-calc.html

In the image below, I just entered the frequency and clicked calculate.


Notice the picture (in stick figure art) is showing you what the answer means:
The top number (4.62 feet) is how long the entire dipole will be, the lower number (2.31 feet) is how long half the dipole will be.

So now, you take some wire (I use galvanized garage door wire from home depot it costs about 20 cents per foot and will last forever) and cut two pieces to the lower number length, 2 feet, 3" and 3/4".

Now you need a piece of coax that has the same characteristic impedance as your transmitter. In my case, my Ramsey FM-25B is 75ohms (most transmitters are 50 ohms however), so I went to the store and purchased a short chunk of 75 ohm coax (RG-6) that would screw into my Ramsey.

You simply peel back the shielding and role it up, and strip off the insulator over the center conductor. Throw away the foil that was wrapped around the center insulator.

I then chop sleeves from power terminals and use these to attach the cut length of galvanized wire to both the center (copper wire) and shield (braided aluminum I believe). I actually don't crimp these, I just solder.

At this point, slide your drip irrigation plastic over, and solder on the ends. I should mention, the ends are a design choice, I like them, they are not required. Make sure that your antenna is not too long with the ends on there. So when i solder those ends on, i cut a 1/3 inch off each wire to make sure my antenna doesn't get longer as a result of the ends soldered on.

Make sure you mount this antenna vertically, as all car antennas are vertically polarized, and it will work best that way. (hanging like a plumb bob, not sideways)

xmus
11-14-2007, 09:47 AM
more

xmus
11-14-2007, 09:49 AM
more

Mudsculpter
11-14-2007, 11:48 AM
GENIOUS,......I'm making one...

Greg in Canby
11-14-2007, 06:17 PM
The exact "how-to" I needed at the exact moment! Perfect timing, thank you.

Wayne J
11-14-2007, 08:01 PM
I had contacted Ramsey when I was building my antenna. I was told (Ramsey Tech) that the transmitter is indeed 50 ohms and the coax needs to be 50 ohm cable. I had a heck of a time finding it. Finally found CB antenna extention coax (53 ohm)
I don't really know why they used 75 ohm connectors and tell you to use 50 coax. :?

klanger
11-14-2007, 08:29 PM
I don't really know why they used 75 ohm connectors and tell you to use 50 coax. :?

So you have to buy their antenna. :)

xmus
11-14-2007, 08:48 PM
I had contacted Ramsey when I was building my antenna. I was told (Ramsey Tech) that the transmitter is indeed 50 ohms and the coax needs to be 50 ohm cable. I had a heck of a time finding it. Finally found CB antenna extention coax (53 ohm)
I don't really know why they used 75 ohm connectors and tell you to use 50 coax. :?

My opinion is the person you spoke to is wrong:
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=TM100
"so that readily available, low cost RG-59 TV coaxial cable can be used for connection. "

RG-59 is 75 ohm.
http://en.wikipedia.org/wiki/RG-59

Wayne J
11-14-2007, 10:14 PM
Something told me to just go ahead and use the 100' of RG59 I had laying here. :(

xmus
11-14-2007, 10:37 PM
Something told me to just go ahead and use the 100' of RG59 I had laying here. :(

The TM-100 is the flagship antenna they recommend, and they go on and on about what a great match it is... well if the input of the TM100 has RG-59 recommended... well then, the transmitter must be 75 ohms.... (or close to it)

nathank
11-14-2007, 11:28 PM
If there is one thing that I simply don't understand, and probably never will, it is how radio waves etc work... so this mini-how to will prove to be very handy!

Just waiting for my TuneCast II to arrive :) Speaking of which, what sort of range have people managed to get out of their TuneCast II's?

xmus
11-14-2007, 11:35 PM
Speaking of which, what sort of range have people managed to get out of their TuneCast II's?

Please start a new thread for that. This thread is pretty clean, and focused. It makes it easier for folks to find stuff if you start a thread called "TuneCast range" for example.

Wayne J
11-14-2007, 11:45 PM
Something told me to just go ahead and use the 100' of RG59 I had laying here. :(

The TM-100 is the flagship antenna they recommend, and they go on and on about what a great match it is... well if the input of the TM100 has RG-59 recommended... well then, the transmitter must be 75 ohms.... (or close to it)

Now here's the kink.... I have the TM100 and the coax used inside is RG174 ;)

So, we have 50 ohm cable inside the antenna and 75 ohm connectors. :roll:

xmus
11-15-2007, 12:01 AM
So, we have 50 ohm cable inside the antenna and 75 ohm connectors

Can you take a picture of the guts? My guess is that there is a balun between the 75ohm and 50ohm portions. If not, then the marketing hype of the TM100, is just that, hype.

DynamoBen
11-15-2007, 12:04 AM
I don't really know why they used 75 ohm connectors and tell you to use 50 coax. :?

While the connector they use is normally connected to 75ohm video cable the connector itself isn't 75 ohm. That being said they could have used something more commonly used with 50 ohm coax. I would assume the reason they didn't is because this style of connector is huge in comparison. Oh well... :roll:

xmus
11-15-2007, 12:14 AM
I would assume the reason they didn't is because this style of connector is huge in comparison. Oh well... :roll:

Someone else already said, and I agree, I think it is mostly to confuse folks into purchasing their antenna! Also, a BNC connector isn't any larger than the type of connector they are using.

wjohn
11-15-2007, 12:16 AM
I don't really know why they used 75 ohm connectors and tell you to use 50 coax. :?

While the connector they use is normally connected to 75ohm video cable the connector itself isn't 75 ohm. l:

Technically it is 75 ohm, (based on the inside and outside diameter etc )

However, the antenna (or the load) will have more effect of the VSWR then the connectors.

Wayne J
11-15-2007, 12:22 AM
So, we have 50 ohm cable inside the antenna and 75 ohm connectors

Can you take a picture of the guts? My guess is that there is a balun between the 75ohm and 50ohm portions. If not, then the marketing hype of the TM100, is just that, hype.

No, the antenna is up on top of the house now. It goes... F connector to RG174 coax, the coax runs to a small pcb, where is runs through a choke, the pcb connects the coax to "ladder wire" with use of a balun.

xmus
11-15-2007, 12:28 AM
No, the antenna is up on top of the house now. It goes... F connector to RG174 coax, the coax runs to a small pcb, where is runs through a choke, the pcb connects the coax to "ladder wire" with use of a balun.


So you are saying they ship the antenna with 50 ohm coax, to connect from the F connector on the back of the Ramsey FM25B, and that 50 ohm coax runs a short distance (how far?) to a PCB where the balun is, and that balun changes the impedance from 50 ohms to 300 ohms (ladder line), then the ladder line runs the "longer" distance up to the roof, and to the antenna?

also, what kind of range are you getting with it on your roof? More than 1/2 mile with "perfect signal" (no fading?)

sandy
11-15-2007, 11:39 AM
If you look at the TM100 manual,
http://www.ramseyelectronics.com/downloads/manuals/TM100.pdf
you see they use the 75 ohm coax, through a ferrite bead (to reduce current flow on the outside of the coax)
then what looks to be a 4:1 balun.
Very common in use to match the 300 ohm twin lead they use for the radiating element. 75 x 4 = 300.
So my guess is that the output IS 75 ohms.
and RG-59 etc should be used.

xmus
11-15-2007, 12:21 PM
If you look at the TM100 manual,
http://www.ramseyelectronics.com/downloads/manuals/TM100.pdf
you see they use the 75 ohm coax, through a ferrite bead (to reduce current flow on the outside of the coax)
then what looks to be a 4:1 balun.
Very common in use to match the 300 ohm twin lead they use for the radiating element. 75 x 4 = 300.
So my guess is that the output IS 75 ohms.
and RG-59 etc should be used.


Excellent :)

Wayne J
11-15-2007, 06:44 PM
I guess no one read the 'parts list'. :?

xmus
11-15-2007, 06:50 PM
Sandy,

ferrite bead, you know, that might further reduce hum :)
I'll try it, and I might add it to the how-to above.

Dave

Wayne J
11-15-2007, 06:50 PM
We really need to have this thread 'split' and 'moved' so we are not hacking your 'how to' (good one I may add)

xmus
11-15-2007, 06:54 PM
We really need to have this thread 'split' and 'moved' so we are not hacking your 'how to' (good one I may add)

I can just change the subject to something else and repost the how-to later.

Brad Riley
11-18-2007, 11:23 AM
Xmus,

Is the length of coax that is connected to the radio a consideration? Could it be longer, say 15 feet?

Brad

xmus
11-18-2007, 12:30 PM
Xmus,

Is the length of coax that is connected to the radio a consideration? Could it be longer, say 15 feet?

Brad

Longer = more loss due to the coax. You can make it as long as you like... just make sure the coax comes into the antenna at a right angle....
With this low of a transmitter power, I made mine really short.
Longer would work... but you would have less effective radiated power...
Just try it :)

sandy
11-18-2007, 02:41 PM
That's the never-ending question.
Which is better,
more power at the lower antenna?
or a bit less power with the antenna raised higher.
It all depends on the coverage area.
Put the antenna at a height where you can see it from your intended coverage area.
Any higher and it will loss power in the coax and in the path back down
(hypotenuse) from the tx to the rx.
Too low, and the signal gets blocked by the surroundings,
trees houses etc, but you putting out a bit better signal where you do see.

look here:

http://www.ocarc.ca/coax.htm

but as a rule of thumb,
if the other antenna can see the your antenna, they should hear it.

xmus
11-18-2007, 02:53 PM
The only thing I would add, is that if you don't require 360 degree coverage then don't put your antenna on top of your house. Why invite harmful interference issues unless you need that kind of coverage?

sandy
11-20-2007, 09:27 AM
Good point Dave,
don't over radiate your signal,
cause if the next neighborhood can see your antenna they can hear it to,
and it might be right on top of somebodies favorite station.
That can make for a less than joyous occasion.
I keep mine up about 15 feet.
I live in a bit of a hollow so that gets me to the edges of my desired coverage, about 200 to 300 feet.

Mudsculpter
11-27-2007, 10:44 PM
I made this dipole and it absolutely works too well. I had some RG6 left over from a dish install and some pieces of lamp cord. When I started driving to check the distance, I know I got about 1500 ft. before I got any static at all. Dang if I had this thing on the roof I would be able to broadcast for miles I believe. I had to hi-tail back and drag it inside the garage some. ........... {taps chin}..... I made a j-pole out of copper tubing a few weeks back,......Hmmmm,..I wonder if......

sandy
12-03-2007, 05:42 PM
I built the 88.1Mhz JPole,
tuned it with the MFJ analyzer
and set it up on top of the (mini) mega tree.
That put it about 20feet in the air in the clear.
I've got good signal up and down the road
as long as you can see the house, you can hear it.
The coax run is about 75 feet of RG8 then 25 feet or RG58.
Lottsa wire with a small signal, but it works !

Macrosill
12-03-2007, 07:06 PM
I built the 88.1Mhz JPole,
........
and set it up on top of the (mini) mega tree.
That put it about 20feet in the air in the clear.
.......

I never thought of that! Very clever, I like it!

xmus
12-03-2007, 07:56 PM
Somewhere here BillAd posted his tunecast inside his "tune to" sign...

here it is
http://www.doityourselfchristmas.com/forum/viewtopic.php?t=341&postdays=0&postorder=asc&start=0

sandy
12-03-2007, 10:38 PM
My current sign isn't 3d so it can't hold the TX,
And the cheapy TX I'm using could drift too much outside.
I like the idea of it in the sign, near the people,
and not broadcasting too far.
I'll try it when I re-do the sign.
I'm lucky this year to even have lights blinking.

Aurbo99
01-04-2008, 07:31 PM
OK, I've read the thread and I'll take a shot at this.

I live in a heavily treed neighborhood, line of sight and clarity of music are my main concerns

I'm using a Belkin 2 transmitter inside a weatherproof enclosure hung 15 feet up in my tree on a limb closest to the road. Its not noticed unless your told its there. I get about 2-300' line of sight which is not quite the entire block. it works but I dont want to have it hanging there swinging in the breeze for all my displays.

99.9 FM is a good choice for me, no stations on it for hundreds of miles as well as no channels on the nearby bands. I'll continue to use this unless someone can tell me a "sweet" channel that I should be using instead..

99.9 FM
My total length will be 5.2700000000000005
Each leg will be 2.6350000000000002

On the Belkin, I will need to create a new BNC connector, to connect to the leg of the Dipole. and that can be of any length I require, correct?

For that connector, the solid copper core to the "ANT" and the braided shielding to common ground somewhere on the pcb?

Does it have to be a BNC connector? could I use an RCA Speaker type jack instead.. I have plenty of those around.

The galvanized garage door wire is used to make the two arms, one crimped to the copper core, the other to the sheilding wire.

Can heat strink tubing be used instead of irrigation drip pipe?

Its hung the way your pic shows in "dipole7.jpg"


Am I on the right wave length? :lol:

kmc123
01-06-2008, 09:40 PM
I did something similar, but "Hardened it up a bit" :)
I have 9 pictures in all of the construction...

kmc123
01-06-2008, 09:41 PM
Picture 2

kmc123
01-06-2008, 09:42 PM
Picture 3

kmc123
01-06-2008, 09:43 PM
Picture 4

kmc123
01-06-2008, 09:43 PM
Picture 5

kmc123
01-06-2008, 09:44 PM
Picture 6

kmc123
01-06-2008, 09:45 PM
Picture 7

kmc123
01-06-2008, 09:45 PM
Picture 8

kmc123
01-06-2008, 09:46 PM
Picture 9

wbuehler
01-06-2008, 10:36 PM
Nice One

Bill

ben
01-06-2008, 10:42 PM
deffinitally making one of those PVC antennas. Then I can mount it outside!

Ben

Virtus
01-22-2008, 09:59 PM
Am I understanding this correctly??
I have a CCrane transmitter with a lousy short little telescoping antennae which doesn't amount to 1/4 wavelength of anything it can transmit. I can take that output to the center of the coax leader to the dipole (or j-pole even) and find a Gnd on the board to connect to the shield and make this thing "sing"?

Brad Riley
01-24-2008, 07:52 PM
Thanks for the Dipole How-To.

I used stranded clothes line wire. The kind with the blue coating so it won't rust. Cut the correct length, attached a piece of RG-59 coax I had lying around. Put it inside 3/4" PVC pipe.
Now I broadcast 700-800- feet before there is any break-up.

Great How-To.

Brad

ptyzzer
01-26-2008, 06:23 PM
My opinion is the person you spoke to is wrong:
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=TM100
"so that readily available, low cost RG-59 TV coaxial cable can be used for connection. "

RG-59 is 75 ohm.
http://en.wikipedia.org/wiki/RG-59

RG-58 is 50 ohm coax and is available from many places. I just Googled RJ58 and got loads of hits, many of them suppliers.