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RF Ground question

Oct 6th 2015, 17:21

KB9YOJ

Joined: Dec 20th 2000, 00:00
Total Topics: 0
Total Posts: 0
Hello-

I am going to be building a Half-square antenna for 20 meters and feeding it in a corner with RG-8x and a bead-balun(choke). The antenna will be hung between my chimney on the house and a tree, suspended by pulleys and counter-weights. My question is regarding an RF ground system.

My shack will be located on the second floor of my house and all system equipment will be DC grounded to a single point for safety. Regarding the RF ground, should I place a screen underneath my carpet, or should I run a quarter-wave radial for (20 meters) along the baseboard? How large does the screen have to be? Should the radial/screen terminate on the station ground buss or extend from my antenna tuner?

Matt
KB9YOJ
Oct 6th 2015, 19:09

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
http://www.antennasbyn6lf.com/2005/04/halfsquare_ante.html
The article first article reference by Rudy N6LF's page suggests that a balun may not be desirable, as it may take a beating when used with a transmatch. This would be indicated by SWR drift.

In theory, a ground plane is a half wavelength on each side, or about 34 feet, which is clearly impractical, which is why it is more common to use radials. Typically, you attach the radial to the point you which you want to be a low impedance. You may need to experiment with what works best for your particular situation. You have more flexibility or options if you run low power versus high power.

Zack Lau W1VT
Oct 6th 2015, 19:55

KB9YOJ

Joined: Dec 20th 2000, 00:00
Total Topics: 0
Total Posts: 0
Thank you Zack for the references. I skimmed the article http://rudys.typepad.com/ant/files/antenna_halfsquare_array.pdf and reviewed the section about current fed half squares. The 'phasing element" will be around 25' to 30' off the ground with the vertical elements hanging down from the corners. This arrangement should exhibit lower ground losses making it easier to use a current choke on the coaxial feedline. Plus, my station is only 100W anyway. And, I can always lay copper on the ground to increase the antenna efficiency(floating counterpoise) and lower the "Earth losses" if needed.

Not looking to make a ground plane as the Half-square doesn't need a radial field. I am inquiring about an RF counterpoise in the shack. Is this even needed for my proposed antenna system (transceiver, ATU, and antenna)?

Matt
KB9YOJ

Oct 6th 2015, 20:43

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
It is difficult to predict whether or not you will need a counterpoise. Most hams just experiment, and add one if there is evidence of excessive common mode current in the station. In the old days, one might get an RF bite from the microphone. These days, one may see evidence of problems on your video monitor when you transmit.

In theory, one could model you station using an antenna modeling program to predict common mode currents, but in many cases, details like the actual electrical wiring around a home are a hidden mystery, unless you have photos of the actual wiring.during construction.

Zack Lau W1VT
Oct 7th 2015, 18:59

KB9YOJ

Joined: Dec 20th 2000, 00:00
Total Topics: 0
Total Posts: 0
Good point. I guess I based my question on the fact that I will be using an unbalanced feedline to a phased vertical array. I'll try it with the choke at the feedpoint and see how she fares without the counterpoise.

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