|[AMRadio] Ladder-line mounting question|
John Coleman ARS WA5BXO
wa5bxo2005 at pctechref.com
Sun Oct 23 14:28:02 EDT 2005
Ed is correct. Better to be safe.
The effect of an impedance bump is a relative thing. Higher
frequencies are affected more. The position of the bump on the line
will affect its size.
Having a piece of metal near a low Z point on the tuned line
will have less effect, perhaps none, than when it is at a high Z point.
High and low Z points are found .25 wavelengths apart. A high Z point
can be found by watching the SWR meter that is in the coax line (between
the tuner and the XMTR) while moving a screw driver or other piece of
metal (that has insulation over it so as not to short the Xmision lines)
up and down the line. This type of testing generally requires two
people, one to move the interfering object and the other to watch the
meter. When the screw driver is near a low Z point it will have very
little effect on the SWR meter but when it is over a high Z point the
SWR meter will change a lot more. If there is very little difference
from one point to the other, then it may be that you have
"inadvertently" matched the load to the line. That's a joke because I
have yet to accomplish this feat with out special pruning. At any rate,
it will not be the same on all frequencies.
Back in the days of the full 1KW input, I had a doublet up and
feed with open wire line. I had the line dropping down from the antenna
above the house and looping under the edge of the roof where it then
came to the balance tuner which was mounted just under the edge of the
roof and against the outside wall. The balance line was insulated #12
wires and laid against the composition shingles just at the edge of the
roof. When I first hooked it all up, it worked fine, but when I turned
up the Variac to run the full KW input I found the in about 30 seconds
the transmitter was out of resonance. The SWR meter was on the pin. I
reduced the voltage and retuned the tuner outside and all was well
again. About 30 seconds after I increased the power it started drifting
out of tune. This time I inspected the antenna and Xmision line to find
that the line had melted the roofing material where it past over the
edge and was now 1 to 2 inches embedded into it and lying against the
roof edge flashing. I placed a piece of Plexiglas under the shingles
and notched the outside edge of the plastic to hold the line wires in
place, spacing them about 4-5 inches away from the roof. The tuning
then was stable.
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Edward B Richards
Sent: Sunday, October 23, 2005 12:06 PM
To: amradio at mailman.qth.net
Cc: amradio at mailman.qth.net; johnson at mailman.qth.net
Subject: Re: [AMRadio] Ladder-line mounting question
Just to be safe why not suspend the ladder line with long plastic cable
ties? This will keep it at least 6 inches from any metal. Good Luck.
73, Ed K6UUZ
On Sun, 23 Oct 2005 12:30:20 -0400 (EDT) John Lawson <jpl15 at panix.com>
> Just got my fence-perimeter NVIS loop installed yesterday (thanks
> the kind slave-labor assistance of Brad, KB7FQR). I'm using 450-ohm
> feedline from the loop back to the tuner in the shack. It comes thru
> hole in the shack wall and I had planned to suspend it along the
> on those large 'bicycle hooks' for the 20 or so feet from the entry
> over to where the Gear all lives.
> It has been suggested that this is a Not Good Thing - that any
> object like that touching the ladder-line will alter the impedance,
> the insulation, etc.
> I can see the insulation concern, but I don't get how a pice of
> steel rod covered with plastic at right-angles to the feedline
> can make much of an impedance bump.
> So I thought to solicit the general wisdom of those of us who
> might know
> the real 411 on this subject, because I'm not the brightest bulb in
> marquee when it comes to HF antennae and feedlines.
> John KB6SCO
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