RadioBanter - View Single Post

Cecil Moore[_2_] · September 1st 08, 08:25 PM posted to rec.radio.amateur.antenna

wrote:
I use an ugly balun with an attic GRrV and do not suffer significant
back-RFI to my transceiver in the regular frequency ranges. I think
that by chosing 1Kohm that may be a bit conservative. In (old)
engineering school, we tended to use an order of magnitude (X10) as
our highly arbitrary cut-off point for impedances that have a
significant effect. You are doing that too but by using 1000 ohms, you
are using (X10 times 2) as your arbitrary cutoff point. Since the
filter is an exponential curve, if you chose 500 ohms instead of 1000
ohms, you might even get a 4 or 5 to 1 frequency range. In my case I
use two different turns chokes so that is why I think I am covered
pretty well. Your information is very interesting; good to see people
are actually measuring things!

Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.
--
73, Cecil http://www.w5dxp.com