"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 04/24/2005
Are you having RF problems in the shack since you put up your HF vertical antenna? This is a common problem with an easy solution. RF energy traveling into your shack on the outside of the coax feed line causes this problem. The solution is adding an "ugly" 1:1 Balun in the coax to choke off the RF energy before it enters your shack.
So what's an "ugly" Balun? Nothing more than a few turns of the coax forming a coil. Don't scramble the turns; they must be side by side for the coax Balun to operate properly. 5 turns or more will be enough for 80-meters and up. Use caution not to coil the coax so tight that it causes the inside conductor to merge into the shield shorting it out. 4" PVC pipe coupler works well and it's cheap. Normally a 1:1 Balun would consist of multiple turns of three wires on a coil form and the simple coax Balun is also multiple turns of three wires when coiled, the center conductor, the inside of the coax shield, and the outside of the coax shield. Now your asking how this could possibly work. RF energy flows on the conductor and not in the conductor. This is the reason silver-plating or copper-clad steel works so well. The RF energy only penetrates the conductor slightly and for plated parts the RF flows entirely in the silver or copper plating and almost never in the conductor below it. In the case of coax this property separates the RF flowing inside the coax from the RF flowing on the outside of the coax.
We can use this problem to an advantage by careful placement of the "ugly" Balun. Most of us would think we would want that Balun at the base of the vertical and try and prevent all RF from flowing on the outside of the coax. In reality this is the worst place you could put it! Why? Simply use the outside of the coax as another radial. Putting the Balun at the base of the antenna would choke off the RF but the vertical element would simply put it there again. The Place to put the Balun would be 1/4-wavelength from the feed point of the vertical on the lowest band the antenna is capable of operating on. If your coax run is 1/2-wavelength or more you will want to place a Balun at each 1/4-wavelength point. You will also want to put a Balun on the feed line just before it enters the shack.
The formula for figuring out where the Balun should be placed is simple. 234 / frequency in MHz * the velocity factor of the coax = length in feet. An example is a Butternut HF6V 80 through 10 meter vertical. As the antenna is capable of operating on 80 meters simply input 234 divided by 3.5 times .75 would equal 50.14 feet assuming your using modern foam type coax with a velocity factor of .75. The HF6V uses a length of 75-ohm coax as a matching transformer. Simply ignore the velocity factor if different from the 50-ohm coax and measure 50.14 feet from the feed point of the vertical for the proper place to coil the coax forming the 1:1 Balun. If your feed line is shorter than a 1/4-wavelength on the lowest band the antenna is capable of operating on, place the Balun on the next highest band where the coax would be longer than 1/4-wavelength. This should cure RF problems associated with RF flowing into your shack on the outside of the verticals feed point.
N4ZOU
WW8X | 2023-01-17 | |
---|---|---|
RF Problems and the HF Vertical Antenna | ||
The following is from the Hustler 5BTV vertical antenna instruction manual: "DECOUPLING YOUR TRANSMISSION LINE Since your coax shield is connected to where a radial would connect, it too will act as a radial. This is not desirable since this imposes additional currents which will detract from the antennas low SWR at the rig end of your coax. To prevent this from occurring, form at the base of the antenna or within eight feet of the base, a coil in your coax. By wrapping your coax in a single layer fashion, ten times around a 6" diameter form, you will form an RF choke in the braid which will isolate these additional currents from your coax. Since your coax (the shield) is in the R.F. near field of your antenna, it will act as a collector of R.F. energy (an antenna). This too is not desirable since this can impose similar additional currents which will register as a higher SWR at the rig. To prevent this, form the same type coil as before at/or within eight feet of your rig. These coils will NOT impede the RF energy contained inside the coax from flowing. It only acts as a high impedance to the undesirable additional currents on the outside braid of your coax." |
G4GZG | 2022-12-24 | |
---|---|---|
RF Problems and the HF Vertical Antenna | ||
This works, end of. Ive used this type of RF choke ( its actually a form of Unun unless you run open wire feed to coax.) at two locations with great success. It has tamed by my end fed that was notorious for RF in the shack. |
VE3CUI | 2022-12-13 | |
---|---|---|
Re: RF Problems and the HF Vertical Antenna | ||
Ah yes, but d'you not see...? It DID work out in my instance --- and the problem was SOLVED...! It's like when I drive my car: I can't "...figure-out" all of the mechanical intricacies of how the thing operates...and for that matter, I don't want to, either. I just know that when I climb in behind the steering wheel and crank the ignition, the thing starts. And that's good enough for me... Just like finally having a ground plane antenna with no RF feedback into the shack, on account of an ad hoc choke I cobbled together--- I guess! :o) Reply to a comment by : W5SO on 2022-12-13 Amen! Just do it - keep trying til you figure it out... Reply to a comment by : VE3CUI on 2022-12-10 Hmmmm... A couple years ago eHam posted an article very similar to yours, in the matter of my getting RF feedback from a multi-band ground plane antenna that I erected close to the shack at our summer cottage --- only my choke consisted of multiple turns of the RG-58 feedline wrapped abound a large ferrite core liberated from the CRT of a defunct TV set... This choke was placed right at the station itself, because of the minimal physical distance involved...but y'know what...? It WORKED --- in fact, it worked VERY well. But you'd never know that such a thing would ever be possible, judging by all of the "Poindexter" flamethrower-armed Hams who went after me, posting claims that it simply could NOT be...! Stay the course, and heed that old Nike(?) advertising slogan --- JUST DO IT...!!! :o) |
W5SO | 2022-12-13 | |
---|---|---|
Re: RF Problems and the HF Vertical Antenna | ||
Amen! Just do it - keep trying til you figure it out... Reply to a comment by : VE3CUI on 2022-12-10 Hmmmm... A couple years ago eHam posted an article very similar to yours, in the matter of my getting RF feedback from a multi-band ground plane antenna that I erected close to the shack at our summer cottage --- only my choke consisted of multiple turns of the RG-58 feedline wrapped abound a large ferrite core liberated from the CRT of a defunct TV set... This choke was placed right at the station itself, because of the minimal physical distance involved...but y'know what...? It WORKED --- in fact, it worked VERY well. But you'd never know that such a thing would ever be possible, judging by all of the "Poindexter" flamethrower-armed Hams who went after me, posting claims that it simply could NOT be...! Stay the course, and heed that old Nike(?) advertising slogan --- JUST DO IT...!!! :o) |
VE3CUI | 2022-12-10 | |
---|---|---|
RF Problems and the HF Vertical Antenna | ||
Hmmmm... A couple years ago eHam posted an article very similar to yours, in the matter of my getting RF feedback from a multi-band ground plane antenna that I erected close to the shack at our summer cottage --- only my choke consisted of multiple turns of the RG-58 feedline wrapped abound a large ferrite core liberated from the CRT of a defunct TV set... This choke was placed right at the station itself, because of the minimal physical distance involved...but y'know what...? It WORKED --- in fact, it worked VERY well. But you'd never know that such a thing would ever be possible, judging by all of the "Poindexter" flamethrower-armed Hams who went after me, posting claims that it simply could NOT be...! Stay the course, and heed that old Nike(?) advertising slogan --- JUST DO IT...!!! :o) |