Choke balun for 20M ground plane

[K3UK]

I have a home brewed 20M ground plane about ready to be erected. I will be feeding it directly with RG8U. The ARRL Antenna book suggest I build a choke balun and provides an example of 6 turns of RG2313 on a 4 inch (diameter) piece of PVC pipe. Can anyone here advise me on what difference there might be if I use RG8U instead of RG-213? Would the amount of turns be essentially the same ? Also, since I have 2 inch diameter PVC pipe already, could I use 2 inch as the diameter for the choke? If so, how many turns would I need for that?

FYI. The antenna will be 1/4 wave above ground , around 17.5 feet. I am initially going to try with just TWO radials and see how it performs. Coax length will be around 75-80 feet.

Andy K3UK

[G3TXQ]

Andy,

I'm not too sure of your coax type-designations over there in the US, but I believe RG8 is essentially the same diameter as RG213. If that's correct, you need the same number of turns. I doubt you'd physically be able to get it around a 2" former. You might also need to think about the insulation material and whether having it bent in a tight radius could cause "migration" problems.

By the way the ARRL Antenna book shows that 12 turns on a 4" former would put the self-resonant frequency close to 20m and give you a much higher choking impedance.

If I'm wrong about the cable diameters - apologies!

Steve

[NA0AA]

If you can bend RG-8 around a 2" pipe you are a better man than I - that's too small - actually, I'd go 6" as a minimum diameter, 8" will also work fine - it's more a mechanical issue. The pipe is just as a form to wind the coil - you don't HAVE to leave it in there. 8 turns should do the job.

Tests have shown that having a neat side-by side wind on a form is MUCH more effective than a scramble wound coil, if that counts for anything.

You could also use a 1:1 UNUN in the application if you have a suitable toroid core for winding one.

You can also use 7-10 snap-on or slide on ferrite beads.

[K3UK]

If you can bend RG-8 around a 2" pipe you are a better man than I - that's too small - actually, I'd go 6" as a minimum diameter, 8" will also work fine - it's more a mechanical issue. The pipe is just as a form to wind the coil - you don't HAVE to leave it in there. 8 turns should do the job.

Tests have shown that having a neat side-by side wind on a form is MUCH more effective than a scramble wound coil, if that counts for anything.

You could also use a 1:1 UNUN in the application if you have a suitable toroid core for winding one.

You can also use 7-10 snap-on or slide on ferrite beads.

Duh ! I never thought about the bending issues

Thanks for the advice folks. Now off to think of what I have available that is 8 inch diameter.

Andy

[G3TXQ]

Andy,

If you use 8" diameter don't go above 6 turns, otherwise 20m will likely be above the self-resonant frequency.

Steve

[K3UK]

I think I will go with a 6,05 inch coffee can

http://www.turnpoint.net/wireless/an...coffeecan.html

[K3UK]

Well, the ground plane is up in the air !

One thing I worry about, the SWR is flat over a WIDE portion of 20M. I cut for 14.100 and SWR 1:1 there but about the same at 14.300. Sign of an issue or to be expected? It hears well. Europe a couple of S2 units over old dipole., which was my mission!

[NA0AA]

Well, the ground plane is up in the air !

One thing I worry about, the SWR is flat over a WIDE portion of 20M. I cut for 14.100 and SWR 1:1 there but about the same at 14.300. Sign of an issue or to be expected? It hears well. Europe a couple of S2 units over old dipole., which was my mission!

What's your vertical element made of? If large diameter tubing, it should be pretty broad-band - although that's enough to make me wonder as well, but you know what, if it's louder than your dipole, I think you know what the answer is.

Might as well run with it, if your tranciever is happy, and people hear you that's what counts right?

[K7MH]

I think I will go with a 6,05 inch coffee can

http://www.turnpoint.net/wireless/an...coffeecan.html

But don't leave the metal coffee can in the coils as something to mount them to.
6 turns of (hopefully) RG-213 wound on a short length of plastic sewer pipe or PVC as a form will give you excellent choking characteristics for 10 through 20 meters.
Not sure why anyone would worry about a good wide bandwidth and very low SWR.

[K3UK]

But don't leave the metal coffee can in the coils as something to mount them to.
6 turns of (hopefully) RG-213 wound on a short length of plastic sewer pipe or PVC as a form will give you excellent choking characteristics for 10 through 20 meters.
Not sure why anyone would worry about a good wide bandwidth and very low SWR.

No, I did not leave metal can in there.

I called CQ and first QSO was with Iceland, second was Cyprus, all barefoot. So it works!

TF3HZ 12-Apr-09 1820Z Iceland 14.076 RST 59


5B4AIF 12-Apr-09 2001Z 14.156 20M SSB RST 56 14.156 Cyprus

215
EI6IZ 12-Apr-09 2211Z 14.004 20M CW 599 599 14.004 20M Ireland