Hi Herb,
Dielectric loading causes insulated wire to have a lower velocity factor than
bare wire.
The velocity factor of your particular wire can be determined experimentally.
This is exactly the same as the dielectric loading that occurs when ice covers
our quads and Yagis (I sure you're forgotten about that in the Virgin Islands!)
.
Ice increases the VSWR and in extreme cases reverses the directivity of a Yagi.
This is exactly the same as what you're experiencing with insulated wire.
All of the loops (reflectors, driven elements, directors) in your quad are now
resonant
at a significantly lower frequency than intended. As far as I know, the only
was to
solve your problem is to shorten all of the wires or replace them with bare
wires.
73
Frank
W3LPL
----- Original Message -----
From: "Herb Schoenbohm" <herbs@vitelcom.net>
To: "topband" <topband@contesting.com>
Sent: Friday, July 26, 2013 10:33:06 PM
Subject: Topband: Somebody mentioned Quads
I need your help on this: Recently my top mast snapped with the passage
of Hurricane Chantal. Amazing the fiberglass three element Tri-band
Cubex Quad survived with the spreaders bent like noodle soup into the
tower guy wires. I took the quad down and decided to restring it since
the #14 bare stranded did not look that good. I replaced all the quad
loop wires with exactly the same measurement but used #14 solid
insulated wire. Everything remained the same but now the quad has the
lowest VSWR at the lower band edge and no longer covers 14 or 21
entirely without a 3:1 VSWR. Ten meters however works fine with a broad
dip at 28.3 which is the same as before. I have made four unanswered
calls to Cubex's voice mail with no reply. This brings up my question
could the velocity factor of bare stranded #14 copper to insulated #14
solid have that much impact on the driven element is frequency and
bandwidth. I can probably restring the quad driven element on the tower
on each band....or....should I simply feed it with a gamma match on each
band. My plan this weekend is to take the MFJ 259B up the tower and
measure what is there on each band.
I bought a Cubex Quad so the entire bands could be covered without the
VSWR default on by Alpha kicking in. On the antenna I use a 3/4 wave
RG-11 matching transformer. I had the also a 3/4 wave RG-11 matching
piece on 15 but when i went to sa direct 52 ohm coax feed I can operate
on CW . However the VSWR shoots up to 2:1 above 21.100. I can not
shorten the 15 meter section anymore with out the whole thing looking
like a Hex beam. This brings up the question: Can I put in a series
capacity (I could start with a variable) small door knob to shorten the
self resonant frequency of the loop. I have never seen this done before
but if I cancel out the residual inductance and have the impedance near
to 50 ohms plus or minus i could reduce the VSWR at least?
I know I should have done all this on the ground before raising the
monster quad....but I thought if I kept all the measurements the same
the antenna would perform the same. Thus your help and suggestions are
needed.
Thanks,
Herb Schoenbohm, KV4FZ
P.S. To get this back on Top Band subject....with the quad I lost nearly
25 per cent of the top loading my A4S provided before on 160 meters.
Something to consider when deciding what to buy.... as my only top
loading now is the quad's 18 foot boom. I still have a 30 foot stinger
on the top but even connecting the quads feed lines with a ground strap
to the boom did not amount to being worth the effort. My tuning box at
the base of the cage feed just does a bit more adjustment for a slightly
less efficient 160 meter vertical.
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Topband Reflector
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Topband Reflector
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