...How does that sound?
Well, let's see...
The feedpoint impedance is about 240 ohms on 20m.
Using VK1OD's transmission line loss calculator here:http://www.vk1od.net/calc/tl/tllc.php
we can see what it looks like through 50' of Wireman 551 ladder line: the
impedance at the balun would be 560-j175 ohms. Total loss is about 0.1dB:
so far, so good.
Divide that by 4, assuming a perfect 4 : 1 balun (which might or might not
be the case.) The coax now sees about 140-j45 ohms.
We can then stick that back in the same calculator for the 50' of RG-213, we
get 36-j44 ohms at the shack end, for an SWR of 2.8 : 1, and a loss of 0.6dB.
The total loss of 0.7dB is probably acceptable, but you'll need a tuner to match
That's the same loss you would have if you put the balun at the antenna feedpoint
and ran 100' of RG-213 to the shack, except that with direct coax feed the
SWR would only be 1.2 : 1.
By contrast, if you lengthened the Wireman 551 to 63 feet, the SWR in the
shack would be 1.2 : 1 and you wouldn't need a tuner on 20m. The total
loss would be perhaps 0.2 to 0.3dB less, but that isn't significant in this case.
That's great if you ONLY work 20m. The antenna itself, with the 4 : 1 balun
at the feedpoint, has resonances at 40m, 20m and 15m, as well as near 10.75
and 17.75 MHz. SWR on 30m is somewhat high (perhaps 8 : 1), but losses
should still be low on 17m where the SWR is about 3 : 1.
If you add a length of ladder line, the resonances shift around. Actually, 50'
isn't too bad, since the SWR on the ladder line is no more than about 2 : 1
on both bands, but by using the 63' length you keep the 40 / 20 / 15m resonances
and actually improve the match on 17m. But the match on 80m, which was
about 2.5 : 1 (at the low end of the band), degrades and is now around 20 : 1.
(To save time I didn't look at 12m or 10m.)
In this case, for operation on 40m, 20m, 17m and 15m, 50' of ladder line to the
balun is usable with a tuner, but 63' gives a lower SWR. On 80m, using the
ladder line between the antenna and the tuner makes the SWR much worse,
and actually increases
losses in the coax compared to putting the balun
at the antenna feedpoint.
For multiband use I've had best results either using ladder line all the way to
the tuner in the shack, or putting the balun at the loop feedpoint and running
coax all the way. Sticking the balun in the middle with random lengths of
ladder line and coax on each end can lead to some quirky impedances and
high losses due to SWR, even when it can be matched with a tuner in the shack.
Sometimes, such as in this case where the loop impedance is well behaved to
start with, you can get away with it if you choose the length of ladder line
Oh, and you don't have to get the loop exactly square: anything close to that
should work. I use electric fence insulators that I can slip around the wire
(rather than having to thread an end through). Typically I lay out the wire on
the ground, add the insulators and ropes to correspond with the available
supports, then hoist the corners in sequence. The insulators can slip along
the wire and equalize the tension (at least somewhat). If you are trying to
maintain a more exact shape, then tie a loop in the wire at each corner and
attach your support rope to that.