RE: Is RG-59 ok for feedline?

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From: rohre (rohre@arlut.utexas.edu)
Date: Thu Sep 28 1995 - 16:31:06 EDT


Actually Paulette,
For a loop, depending whether it is fed in one corner, or the center of a
side; the impedance will be closer to that of RG-59 than RG-58. The 58 is 50
ohm cable, the 59 is 75 ohm. If the loop were fed in the center of one side,
you could expect 150 to 200 ohms antenna feed point impedance; thus the 75 ohm
cable might be a mis match of 2:1 SWR, while the 50 ohm cable would be 3.5:1
SWR. This is for a high loop, and in reality, ground being near and under it
will modify the antenna input impedance of the loop.

If you can borrow an Autek RF-1 or MFJ antenna analyzer, you can attempt to
characterize your loop input impedance. I say "attempt" because you may be
putting the feed point higher than you can reach with a ladder to attach the
analyzer right at the feed. What you could do is temporarily pull the loop up
high enough above the roof that you can still touch the center of one side, or
a corner. Place an insulator both places, to open the loop. With the
analyzer set to measure impedance or SWR, short the corner opening and measure
the center feed impedance. Now reverse the process, putting a jumper at the
center, and measure the corner feed impedance. Use whichever is closer to the
75 ohms, if it is under 2:1 SWR, and you are using the RG-59 coax. You could
also calculate an exact coax length that would transform antenna feed point
impedance to 50 ohms at your rig, but that would be good for 40M and no other
band. You did not say, but if you have a tuner, you could try the loop on
higher bands.

IF your run of coax to the rig is not very long, say well under 100 feet, you
could just feed whichever point is convenient, and use a tuner, tuning both
the coax and the loop to match to the 50 ohms your transceiver wants to see.
You would need an SWR meter in line to the tuner, and a tuner for unbalanced
line.

Now I can hear the howls of the purists. Actually, the loop IS a balanced
feed antenna, and in THEORY you should be feeding it with some kind of open
wire line, or twin lead, both of which are balanced lines. Now if you can't
do that, you could use a 2:1 balun at the center of a side, and use the RG-59
for its 75 ohms would then be stepped up to a pretty good match to a practical
elevated loop. The balun at the feed point provides an insulator, and the
transfer from balanced antenna to unbalanced coax. If you have a 4:1 balun,
lying around, that might work, but less well. BUT, since this is 40 Meters we
are talking about, you probably can get pretty good results by direct coax
feed, if you have a tuner, and no one can hear the difference on the air, for
the horizontal loops most hams can put up. ( That is, above a house, rather
than completely over open ground and a half wave high.) Part of the thrills
of amateur radio is making something do with what we have available at the
time. If you find you have too much RF on the outside of the coax, or high
SWR in the shack that can't be tuned out of the unbalanced feed, then you
could go to the expense of the balun, or open wire line feed.

Commercial modern rigs have SWR sensing circuits to cut back power or shut
down if the antenna feed is too far off what the rig wants to see. The kit
QRP rigs have a zener on the collector of the final transistor to protect it
for the same reasons. Thus, nothing bad will happen up to 2:1 SWR, and maybe
more, if the rig has working protective circuitry. Your power out may suffer
a little on a commercial rig that lowers power automatically above 3:1 SWR.

The wave lengths are so long at 80 and 40 M, that unbalancing a dipole, or a
loop may not detract from its operation. Another trick for the loop with
direct coax feed, is to just coil about 6 turns of the coax at the feed point
if no balun is used; and lead the feedline away at right angles to the wire of
the loop. This "cable choke" will choke RF that would tend to flow back over
the outside of the coax shield toward the rig.(That is where RF in the shack
could originate.)

Notice again for the purists, I have not approached this from a ground up theoretical
this is best and the only way to do it; because it appears from the post that
certain materials were at hand; and perhaps no balun; so how do we get this
working right away? Yes, doing it the "book" way may give the best results;
but I think we learn by doing, and no harm will result from trying what is
available, given that we monitor the SWR, and try to make the rig happy.

Good luck with your loop, and let us know if you can measure the corner vs.
center of a leg what you find, for the elevation you choose. (And indicate if
part or all of the loop is over the house.) Another way to measure it at full
elevation is to use an exact half wave of coax for 40M, and measure at that
frequency, with the loop cut for that frequency, and then what you measure at
the coax end in the shack will be reflected feed impedance. But you may have
an upstairs shack, and not need that much feed line, etc.

72, Stuart K5KVH
rohre@arlut.utexas.edu


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