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AMfone
Preventing Coax Feedline Radiation

Preventing Feedline Radiation

submitted by Tom Cathey K1JJ



Someone on AMfone's "AM Forum" asked about feeding an antenna with coax and how to handle feedline radiation issues. As you may know, feedline radiation can distort an antenna’s pattern into something that is much different than the predicted model. Here’s my reply:

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For a Yagi, consider using a TRUE 4:1 coaxial 1/2 wave BALUN [BAL-UN] at the feed point. Real EZ to make. 11' 4" of RG-213 [0.66 velocity factor] is what's needed coiled at the feed point for 28.75 mhz . It produces a true transition from unbalanced coax to a balanced driven element. I use them here on all Yagis and other wire antennas. This is assuming you can work with the 4:1 transformation. Many low input impedance Yagi designs use a T-Match that is a step up. They can be brought back down to 50 ohms with the 4:1 coaxial balun. This balun is good for only one band at a time. Use the formula 492/ freq * .66 (coax vel factor) to get the 1/2 wave balun length in feet.

Here's how you connect it: You have two ends of an 11' 4" piece of RG-213 [coax balun] and one end from the RG-213 feedline. [Or whatever type coax you use]. At the driven element feed point, tie the three shields together. Then connect one inner conductor from the 11' coax to one leg of the driven element. Then tie the other inner conductor of the 11' coax to both the feedline inner and the other leg of the element. The coaxial 11' balun can be coiled up and mounted right at the feed point. Notice none of the coax shields ever touch the element's legs. It works by feeding each leg 180 degrees out of phase as it should be, with floating inner conductors.

The balun's bandwidth is greater than the Yagi, so no limitation there. In contrast, using a coax coiled choke and/or beads works FB, but gives a 1:1 bal - un transformation, which is better for a simple dipole. In addition, it is said that ferrite can become damaged [permanently saturated] by a nearby lightning pulse, whereas a coax is immune to nearby stuff. I've used ferrite and never had this happen, but just wanted to mention it.




Question: Can use a Radio Works dipole current balun in place of the 11' 4" (for 10M) of RG-213?

Hi Ed,

Does it use a torroidal transformer balun, ie, wire wound on the balun in a true transformation, or does it work as a choke?

Transformation toroid baluns are used all the time. The StackMatch is a good example where toroids are used up to 6M with good results for matching unbalanced to balanced systems. Chokes are a different story, but still work "OK", but not perfectly.

I would rate the effectiveness of reducing feedline radiation in order - starting with the best method. This is only MY opinion:

1) BEST: True COAXIAL 1/2 wave transformation balun cut for the operating freq - measured with an ant analyzer. [bullet proof to everything - EMP, power saturation - limited by melting at 10KW power level... T
rue transformer- no reason for chokes if used. Disadvantage: Good for one band only - 4:1 ratio. Not good for a directly fed dipole.


All the following can be used for single dipoles too:


2) REAL torroid transformer balun. Real transformer with wire windings - and allows for a 1:1, 1.5:1, 2:1 step up, etc... flexible for matching. But, EMP susceptible. Advantage: Broadband for 1.8-10M with some designs.

3) Choke methods: Use BOTH Ferrite Beads AND coaxial coil [hank of turns, not a transformer above] A band-aid approach since the coax is still unbalanced and the dipole is balanced... no transformation. Broadband.

4) Lots of big ferrite beads (IF they are the proper permeability for the freq and large enuff not to saturate) They are susceptible to permanent EMP saturation, however. Broadband.

5) Coiled Coax turns alone at the feed point. Broadband, but coiled hank tends to have inter-cap problems above 40M. Coil coax turns on a PVC form for a better coil above 40M. 160-40M OK to coil in a hank and tape.

6) W2AU choke ferrite type "baluns". Smoke city on QRO AM.

7) Direct feed, no choke, with feedline coming away at perfect 90 degrees.

They all "work" - just a matter how type A you wanna be. And, individual physical layouts will change results. Houses, other ants, departure from straight and flat textbook dipole form, etc.

Will there be much difference between #2 and #7 for a dipole on 75M? Probably not. The #7 pattern may be skewed slightly, but the on-the-air results will be good either way. Where it really begins to matter is when you have a directional antenna, like a Yagi that wants to see a nice pattern - nice front to back and clean frontal lobe. But, where it really is critical to have near zero feedline radiation is when stacking antennas/Yagis. These imbalances can wreak havoc on the phase relationship between antennas and can often make a good system perform poorly.

But my philosophy has been, if it takes only a small EXTRA effort to make something near "perfect", why not? A db here, a db there - after a while they add up and give an edge over the station that let the smaller details pass by.


T