Assistance with Half-wave Dipole Baluns in the FM Band

[FourierFan]

Hello!

If there is anyone out there who would be willing to assist me with a balun design for an FM transmitter antenna I would appreciate it.

I have a transmitter that is power output adjustable between 1mW and 25mW and was hoping to make the antenna as efficient as possible, thus running the transmitter with as little power as possible while still being compliant with the laws in my part of the world for a license-exempt broadcast.

I've found many websites in my searches that discuss baluns, but it seems the breakdown of information from google searching falls into the following categories:

90% of sites say "you should use a balun to balance the antenna" or equivalent statements
9% of sites try to sell an antenna with a balun as part of a kit
0.99% that give generic instructions for making a balun (e.g. "wrap the coax 5 turns", "wrap some wire around a ferrite core", etc)
1 website that gave specific instructions on using a piece of λ/2 coax to make a balun but in the context of a "closed loop" dipole(?).

If anyone has any specific instructions for making a balun that will work with a basic half-wave dipole I would appreciate it.

Cheers!

 

[Nigel Goodwin]

No need for it, and point in it - basically the output of the transmitter should be unbalanced and 75 or 50 ohms. The impedance of a dipole is the same 75 or 50 ohms, and you connect it via 75 or 50 ohm coaxial cable. Baluns only come in for folded dipoles, whose impedance is 300 ohms, so require a balun to convert it to 75 ohms.

 

[MikeMl]

You should only use a balun if it is required. If your transmitter was designed for a nominal 50Ω coax load, then there are lots of antenna designs that have an feed-point impedance of 50Ω, so coax cable can be used to connect from transmitter to antenna, and no balun is required. The J-Pole or GroundPlane vertical antennas are examples for which no balun is required. If you want to feed a horizontally-polarized Simple Dipole, a 1:1 balun can be used to transform the intrinsically unbalanced 50Ω coax to the balanced feed at the center of a dipole. A Folded Dipole would require a 1:4 balun

 

[FourierFan]

Thanks for the replies, I appreciate it.

So the upshot is if my feed-point impedance is 50Ω, I'm using the RG58 (50 ohm impedance) coax to connect to an antenna, and my antenna is a basic λ/2 (vertical) dipole that can be found everywhere on the web, then no balun is required.

Do I have that right?

If that's the case, why does Ramsey in their dipole antenna kit include a balun as part of their "Tru-Match FM Broadcast Antenna Kit" kit? Is it a marketing ploy, the fact that its a vertically-polarized antenna, or to make the antenna more flexible with respect to transmitter applications?

 

[MikeMl]

A λ/2 center fed dipole mounted several λ from other metallic objects, and if the feedline is brought off at right angles from the center of the dipole, has a theoretical balanced feedpoint impedance of 73Ω.

When feeding it with 50Ω coax (unbalanced), you can do various things:

1. Hook the coax center conductor to one leg, and the shield to the other. Practically this works, and it is done all the time. It has two problems. Due to the unbalanced feed line, currents in the dipole legs are unbalanced, therefore the antenna has asymmetry in its radiation pattern, and the current on the outside of the coax also radiates in the vertical plane, adding to the pattern distortion. The losses attributable to the slight mismatch of connecting a 50Ω feedline to a 73Ω antenna are so slight as to be insignificant.

2. Use 70Ω CATV coax, and diddle the transmitter to match 73Ω. You still have the unbalance dipole currents/feed line radiation, but lower SWR losses.

3. Use a 1:1 balun between the 50Ω coax and the dipole. This straightens out the radiation pattern and eliminates the coax radiation. However, the balun loss wipes out any improvement. The mismatch loss in the coax is uneffected.

4. Use 75Ω balanced twin lead to feed the antenna. This requires moving the balun to the transmitter end of the feedline. Most transmitters can be retuned to match 70Ω by adjusting the Π-network output tuning. Gets rid of the feedline radiation, and balances the currents in the dipole. Twin lead has lower losses than coax. This is the preferred solution.

5. Use 300Ω balanced twin lead to feed a "folded dipole" with a 300Ω feed-point impedance. Need a 1:4 balun at the transmitter. Even lower feedline losses. This is the way analog TV antennas used to work.

Any "improvement" in range by choosing method 1-5 is apt to be so slight (~1 or ~2db) that you will be hard pressed to notice the difference.

 

[FourierFan]

Thank you for taking time to put together such a detailed reply. I truly appreciate it.

 

[MikeMl]

(This was asked in a PM from Matt):

I had a question that I thought you would be able to answer. I have a very low powered FM transmitter, and I would like to make a simple, yet efficient antenna for it. The antenna output is a BNC connector, and it states that the output expects 50Ω.

I was going to make a 1/2 λ dipole, fed via twin lead.

My question is this: would using this adapter be a good way to connect the BNC connection to the twin lead for this setup?
https://www.amazon.com/gp/product/B00...A385A0XNQBW8HY

Any advice appreciated.

Thanks!

Matt

_______________________________________________________________________

Get a 300:75 Ohm TV balun. These where the little black things that used to come with every TV sold in the 1970-1980s. There are millions of these things sitting in people's drawers...

It will have a Type-F Coax fitting on one side. Connect that to the 50Ω spigot on your transmitter. You may have to find a BNC to Type F adapter, or change the connector on your transmitter to a Type F Female.

The other side has screw terminals. Connect those to a 300 Ohm twinlead "feed line" of any reasonable length. Make a "folded dipole" out of another piece of 300 Ohm twin lead, length (in meters) = 142.5 / f (where f is in MHz).

Another option: Get the other type of 300:75 Ω Balun, the kind that has the pigtails on the 300Ω side. Use CATV 70Ω coax to run from the transmitter to the dipole. Make the same folded dipole as above but connect the balun at the feed point.

Orient the folded dipole to match the polarization of the receiving antenna (vertical or horizontal). If you are trying to improve range to a specific receiver location, any improvement in the receiving antenna (including using a directional Yagi type beam antenna) will improve the signal-to-noise at the receiver.

 

[BustedFlush]

Thanks Mike, for the answer and for re-posting the question.

For my own knowledge, can you explain why that solution would be better than the adapter from Amazon? I do have a few of those old TV baluns, but I was trying to reduce the number of adapters, with the reasoning that the fewer the better, and this way it would seem I have balanced output from the get-go?

 

[MikeMl]

This adapter is nothing but a direct connection; center pin of the BNC to the red binding post, outside to black. I have several of them around here.

There is no impedance transformation in that adapter. If you want to use 300Ω twin-lead as your feedline, then you must use a 4:1 balun at the transmitter. If you are willing to use 50 or 70Ω coax as a feedline, then the you can use a simple dipole (not folded) at the top, or you can use the 1:4 balun to feed the folded dipole.

You could use that adapter if you can find some 75Ω twin-lead, and feed that to a simple dipole, but 75Ω twinlead is quite rare.