Paul, I fully agree with Frank.
You should indeed switch to 300 ohm or better yet 450 ohm.
Once you have done that, you can play with adding various lengths
of extra feedline (which will change the impedance seen at the
matchbox). Without a modeling program, it's trial and error, but
usually not too difficult to find a length that solves the
problem. The bigger problem will be switching it in and out when
you switch bands. You might even get lucky and find a length
that works for all bands.
Another suggestion, and you were close, is to shorten the loop to
about 40.5m total length. For 40m and above, make it a closed
loop, but for 80m, open the loop at the far end. It then is
really just a dipole, folded in a square. Just be sure you open
it exactly in the middle - otherwise it will be off-center-fed,
which is undesirable. I've had good luck on 80m using this
arrangement on a small lot.
Finally, check to see if the MFJ-949 uses a current balun. I
don't know. If it does not (i.e., if it's a voltage balun) then
I recommend using an external 4:1 current balun. Radio Works has
an excellent balun for this purpose (called the "Remote Balun").
73
Rick
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com]On Behalf Of holladayfd
Sent: Monday, January 05, 2004 12:06 AM
To: tentec@contesting.com
Subject: Re: [TenTec] Not TT - Wire antenna question.
Paul,
I have used the same basic set-up here with very good results.
There were
two changes however which I found necessary. First, winding
RG-58 on a
small diameter core will work for a few days but after a short
time, the
center conductor will distort the poly insulation and change both
impedence
and capacitance of the coax. It would be better to use a few
turns on a
poly form of about 4 inch diameter. Actually, for multiband use,
ladder
line of either 300 or 450 ohm impedence would be much better and
easier to
match. Try WWW.cebik.com for more info.
73,
Frank, K4VMO
----- Original Message -----
From: "Paul" <gw7lhi@hotmail.com>
To: "Ten-Tec List" <tentec@contesting.com>
Sent: Sunday, January 04, 2004 4:39 PM
Subject: [TenTec] Not TT - Wire antenna question.
Hello all, thanks for looking
As folk were discussing wire antennas, please forgive another non
T-T
specific post!
I have a fair amount of experience with beam antennas on HF /
VHF, and
colinears on VHF but very little practical experience and know
how with wire
antennas.
I finally got round to putting up a horizontal loop for HF use.
It is a slightly distorted rectangle at about 23ft AGL, and is
supported by
4 fibreglass telescopic fishing rods.
The total length of wire in the loop is 48m or 156ft, the
rectangle has 17m
sides and 7m ends
The wire is 1mm diameter with 19 strands.. it's very thin and
fireproof,
also tough.. I cannot break it with bare hands. Made by RayChem.
The feed arrangements ( don't laugh now! ) are an 8ft long piece
of RG58 and
terminal block to the middle of a 7m end. I did consider using
some 300 ohm
twin feeder, but thought that as it is such a short run to the
shack that
co-ax would do for a test. I have wound 12 turns of the RG58
around a 3/8
inch ferrite rod as an RF choke, this is located right at the
feedpoint.
The choke was an afterthought to cure the excess RF in the shack
on 80m.
My shack is upstairs, the nearest dirt is 20ft of wire away at
least. I have
no RF problems with the vertical, but that antenna is about 30ft
away from
the shack.
The antenna loads up on all HF bands using an MFJ 949e tuner,
although it is
harder to achieve a good match on 80m.. it seems particularly
reactive on
80.
To my surprise, it happily loads up on 160m.. haven't tried
calling anyone
yet.
Good results on 40m daytime UK signals, most reports said the
loop was 10 or
more dB better than my Butternut HF6 vertical which is ground
mounted with
buried radials.
I can see a good dip on a GDO at around 6.8MHz, which agrees with
an SWR of
2.5:1 at 7.0MHz as indicated on the ATU metering.
Now for the questions..
Is there anything I can do about the reactivity on 80m?
Should I swap the co-ax for 300 ohm twin feeder?
Would it be worth shortening the loop slightly to bring the
natural
resonance up to 40m?
Or is there another "magic number" to multiband the loop better?
I like to talk local and work DX, would it be worth adding
vertical segments
at each corner, running down the 6m long fishing rods?
Or maybe I should consider another form of wire antenna
altogether? I picked
the loop because I know a few others using them to good effect. I
can only
really have 1 wire antenna installed at a time.
Any other good ideas welcome, the main bands I want the antenna
to be
useable on are 160 / 80 / 40m, although 40 and higher are a bonus
really.
Any good websites also welcome.
My email address is valid.
A very happy 2004 to all!
Paul MW0CDO.
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