Topband: Elementary Inverted L questions

[Rob Atkinson]

>I have been trying to replace my inverted L, which broke a few weeks ago, and am having all sorts of puzzlements. Please be kind - I was a history major. 1.  The inverted L broke at the turn, so I went looking for sturdier >wire.  I wound up with some #14 insulated recommended by our local RF maven,

I advise ordering a roll of number 14 bare, 7 strand hard drawn copper
wire.   Put up a continuous length of it.  It will be less likely to
break.   On-ground radials can be solid soft wire.  Not the vertical.

> and put up 140 feet initially, figuring I would prune it *up* into the CW part of the band.  Imagine my surprise when it showed up resonating at 1977 KHz (on an AA-55 Zoom).

Your plan is okay, but something is causing a problem.  Firstly, you
need     to     put    down      more      radials.

>  Does insulation have such a profound effect on velocity factor? 2.  My feedpoint arrangement has 17 turns of RG-400 as a common mode choke, with the radial field connected to the RG-400 shield at the antenna end.

Ditch the choke and put your measuring instrument right into the
driven wire and ground system.    If you have 60 or 70 radials, you
don't need that stupid choke.

> Initially I tried measuring the resonance without the radial connection.  No resonance and infinite SWR.  I connected the radial field (8 X 70-foot radials on the ground) and that's when I saw the 1977 KHz resonance (X
> crossing zero).  SWR at minimum was above 2:1.  Does this make sense?  I know the radial field is inadequate for good performance, but...

The vswr only 2:1 makes sense because you have a poor antenna.  It
should be 4 to 5 to one minimally on some frequency on an analyzer
that is normalized for a 50 ohm load, IF you have a decent ground
system.  With few radials Z is higher up and closer to 50 ohms instead
of 10 to 15.  This also depends on the vertical height.  I'm assuming
a typical inverted L that's around 50 feet vertical.      Analyzers
put an extremely low amount of power into whatever they're testing.
The highest power unit I know of is the MFJ 259 family which puts out
around 20 mW.   Poor continuity such as corroded surfaces at hardware
joints or cold solder joints or contact surfaces such as banana plugs
and jacks will be high enough R to throw off the instrument.  Put 100
w. into the antenna for a few seconds to blow through these problems
and check it again without the choke.  Ideally an RF source should not
fold back.  The old Ten Tec rigs were good for that.  Better still is
a tube rig like a DX100.  It will dump 100 w. and not give a flip
about the load Z.  Hit it for a second or two with whatever you have.
The excited wire may be broken, or ground system bonding flawed, or
something else.

Example case:  I have an inverted L 50 feet high that's held up with a
guyed 3" o.d. aluminum mast that's 50 feet high in two sections.  The
mast has one point at 31 feet where the sections are bolted together
and guyed.  The top 19 feet is free standing.  Naturally there's some
interaction between it and the driven inverted L wire.   When I put it
up, I stupidly neglected to put Noalox on the joint at 31 feet.  Now,
on extremely windy days and nights, the mast bends and flexes and that
joint becomes intermittent.  This affects the inverted L Z.  My L
network matches the antenna to 50 ohms under normal conditions but
when the wind gusts up continually to 50 mph or more, I see the vswr
jump up to 3 to 1 roughly, as if a switch is being flipped.   I have
to tilt the mast over and disassemble that coupling, clean and
reassemble it with Noalox.

73
Rob
K5UJ