Earlier this summer I pulled a Kt34XA and 2 element KLM 40 mtr yagi off
the tower for refurbishing and to eliminate two problems. The first
problem was high vswr on the KT34XA on 15, and most TT readers saw the
resolution of that one.
The second problem was somewhat high swr on 40, when the booms of the two
antennas were 90 degrees from each other. The 2:1 swr didn't seem to
make the antenna play any worse, but as you know, mentally you are louder
with 1:1 than with 2:1!
I was also anxious to see what sort of interaction there would be between
the XA and the 40, when both are working correctly and stacked eight feet
apart. I was concerned that the 40 would tear up 15.
A little info on the 40. I bought it used about a year ago, and decided
to put it up and see what it would do, without chaning anything or
cleaning it up. It's an older antenna that still used the "maxi" match
with a big transmitting capacitor instead of the quarter wave matching
scheme currently used. It replaced a KLM rotatable dipole at 63 feet in
the air. My perception was that the yagi did indeed work better than the
dipole.
Well, I found a couple of interesting things while taking the 40 apart.
First most of the tube to tube joints were terrible. As I pulled them
apart, some sounded as if they had sand in the joints, and all of them
produced copious amounts of aluminum oxide. I could tell that some form
of conductive paste was used when it was put together, but a cleanup was
long overdue.
Most of you already know this, but if you are putting a new antenna
together that uses the aluminum clamps in place of hose clamps, do
yourself a favor and invest in stainless steel hose clamps. I had a
dickens of a time getting a couple of elements apart where the bolt in
the aluminum clamp had "dimpled" the outer tube into the inner one. I
finally got them apart by inserting a piece of aluminum tube the same
size as the smaller one through the larger tube and driving the smaller
tube with a hammer.
As i was cleaning the 130 inch long linear loading tubes, I got one of
them to whipping back and forth a bit. It broke in half at the middle!
While inspecting it, I discovered that there was a nest of some small
critters at the point that it broke. The dead insects appeared to create
a corrosive substance that ate away the inside of the tube. KLM
recommends against caps on the ends of tubes, to allow water to drain,
but after seeing this, what does one do about keeping bugs out?
I used a chemical designed to clean coils in air conditioning units to
clean all the tubes. I also used one of the plastic scrubbers on
particularly difficult areas.
I assembled the antenna with the boom strapped to the tower as I did with
the XA. I compromised between low fone and cw settings by using the cw
dimensions minus one inch on each half element. I put it up with the old
matching section to make sure I was in the ball park, and then changed to
the quarter wave matching section. It looked good.
Thanks to N8UG for information on the velocity factor of RG59 MATV cable.
And thanks to K6LL for info on cutting the quarter wave section to
rersonance. The trick in this case is to put a tee connector on a dummy
load. Connect one side to the rf source and the other to the piece of
cable to be used. Short the far end of the cable, and determine where
minimum swr is. I found the AEA antenna analyzer to be a great help, but
there's nothing wrong with using your transmitter and an swr bridge.
Start cutting cable from the end, shorting it and measuring swr until you
have 1.0:1 at the desired frequency. The quarter wave matching sections
are very broad. Incidently you can do the same thing with half wave
sections, only leave the end open instead of shorting. There's some info
on this in ON4UN's low band dx book.
After moving the antenna to the top of the mast with the boom 90 degrees
from the XA, the resonant frequency is around 7080khz. SWR remains below
2:1 between 7000 and 7225. Power doesn't start folding back until I hit
7260.
I'm not sure whether cleaning up the joints or replacing the matching
system corrected the swr problem when the booms were 90 degrees apart. I
tend to believe it was the matchng section, because the driven element
was electrically connected to the boom with the old system, and it's not
with the quarter wave section. I subscribe to W3LPL's practice of
isolating elements from the boom.
What sort of interaction am I seeing between the two antennas? Well, let
me start by saying that before I found the real problem with the XA on
15, I adjusted the reflector to extend the low end of the band. When I
found the real problem, the low end was extended even further. Expecting
that the 40 would tend to pull resonance up on 15, I just left the
reflector as it was. SWR at the low end of 15 was about 1.4:1 without the
40 in place. Once the 40 was placed eight feet above the XA, the swr on
15 rose to 1.7:1at the low end of the ban.
How do the antennas work? Well, I'm extremely pleased. The only band I
notice any interaction on is 15, but it doesn't appear to effect
performance. I haven't had a chance to closely check f/b with a local
source, but in swinging them back and forth on each band with a variety
of stations, it appears that both antennas are near or at spec. The ssb
Sprint and VK9 expedition have provided a competitive playfield to
analyze performance. Things are cooking. I finished putting all the low
band wires/N4KG specials/beverages up over the weekend, and am prepared
to do battle!
73 de Larry K7SV
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