At the minimum you would want to use a high power balun to interface the
unballanced coax with the ballanced 450 ohm line. As to why type or impediance
of balun, that would be determined by the impediance of the 450 ohm line at the
operating frequency you intend to operate. I ran some numbers using EZNEC +5.0
for a 135' inverted V dipole 40' high at the center and 10' high at each end.
I modeled the 90' of 450 ohm line you said you could use. Here is what I got:
Freq. R J
3.8 46 -59
7.2 31 -71
14.2 170 +677
21.3 159 -486
28.5 130 +254
AS you can see the R ohms would work for a 1:1 balun on 80m and 40m but a 2 or
3:1 balun would work better on 20m, 15m and 10m. The J ohms are reactance
caused by the length of the antenna not being exact multipuls of the
frequencies I used on my calculations. For this reason, at home, I bring the
450 ohm line in through the window and use a Johnson KW matchbox to provide an
EXACT match to the 50 ohm line at any frequency. You could adjust the antenna
length to give a lower J at the operating frequencies if you are not able to
use a good ballance antennal tuner.
I do not know what stress you would place on the balun if you tried to use one
size to fit all but ,what the heck, its only a hobby....
Hope this helps,
---- KI9A@aol.com wrote:
> After some serious tree trimming, I now have room to fit a full sized wire
> antenna where my G5RV is right now. I am thinking about a 135' open wire fed
> dipole, as multi bands are needed.
> Question is this- there is now way I can get 450 ohm feedline into the
> shack. The best I can do is about 30' of RG213, then the rest 450 ohm (
> 90'). Is this going to be a waste of my time, or is it worth it? I'd
> use a balun to change over to 50 ohm feedline.
> 73- Chuck KI9A
> **************It's only a deal if it's where you want to go. Find your travel
> deal here.
> TowerTalk mailing list
TowerTalk mailing list