I did not see the original message on this subject - but I rebuilt a KT34 to
KT34M2 a couple years ago. It was impossible to get 15 meter performance -
correct - F/B was in order of 6 to 9 db in CW end of band . I had a couple
conversations with K6MYC as well as Msquared and - just could not get there.
Finally started measuring element resonances using a battery operated
AIM4170 at the feed point with RS232 data line choked off every few feet -
using a wooden gantry with rope that would raise the elements vertically so
the bottom end was about 8 feet off ground. I could move the RS232 line all
around and not chnage measurements - it came off at almost right angles for
about 8 feet then slanted down )
This showed the reflector was resonant inside the low end of 15 meters . I
started trying to extend the 15 meter dimensions to get that resonance
down to something like 20900 or so . This was impossible with the original
KT34 tubing at max length . I then started adding length to the 15 meter
trap tubing and finally got it moved down to 20900 and things started
falling in place. (This antenna had NEVER been quite right on 15 meters
with SWR climbing very fast below 21030 or so and F/B was poor at low end).
I was able to achieve F/B in the 17 - 20 db region across 15 meters and SWR
under 1.5:1 across 15. (this was on a 6 foot tripod on peak of roof - whne I
put it on tower at 42 feet swr went up a tad but F/B stayed .).
At this point I might mention that I spent about 3 weeks chasing my tail
after I first got the element resonances down where I thought they should
at least be for a start.. My F/B measurements were made with a friend as a
signal source about 1.5 miles away and due north of me. The dB quoted were
with keeping a constant signal level at the receiver with a step
attenuator - not any S meter readings involved. I would get F/B in the 20 db
region and it would either be tilted tremendously from one end to the other
of the band - and worse yet the next day the F/B would not repeat at all.
Finally one day I decided to rotate my 40 meter dipole (which was 35 feet
above the KT34 and about 10 feet to the side ) and the F/B would go all
over the place both in frequency and magnitude. I then nulled the 15 meter
signal from the 40 meter dipole by putting it end on to the signal source
and all the screwball variations went away.
I bought new 3/8 tubing for the 15 meter traps (the supplier let me go back
in stacks with micrometer and pick tubes that were very close to exactly
0.375) to get rid of my fabricated extensions and .was able to get things
looking pretty much as M2 showed - not quite the 10 meter BW they show but
was ok to about 29000 and F/B was in 17 - 20 db range on 10. For some reason
that I have not been able to figure out - the measured 20 meter F/B using
the local signal source was never much better than 12- 15 dB - but on
distant signals it has shown over 20 dB many times. The element resonance
looked reasonable so I decided to stop at that point .
Now what was the problem ???? I found the 3/4 inch trap capacitor tubing was
almost 3 to 4 mils large ID and my 3/8 tubing was about a half mil or so
over 3/8 so the 15 meter trap capacitors were quite low in capacitance
compared to that which you would calculate and thus required a lot more
length . (I did try inserting some teflon capacitor slugs but that was a
real pain to handle so gave up on that .) I can't find my notes right now
but I recall the traps at nominal should have been about 41 pfd plus the
capacitance of the end caps and I was about 1.5 - 2 pfd low.
I was never able to find any other element resonances data for KT34 -
several for KT34XA - but K6MYC did send me some of his notes for early KT34
development and I ended up pretrty close to his .
The antenna has been up almost 4 years now and other than a couple
directions it is better than 1.5:1 across the bands thru about 70 feet of
RG213 and a RATPACK antenna switch. and residual SWR of ant tuner in
bypass - and worst case is about 2:1 on 20 meters and only for a few
degrees due south (have never figured that out - probably the power lines in
the alley behind the house across the street which are about 7 feet higher
than my lot and thus pretty close to right at the KT34 height) .I should
also mention these data are using a new M2 4:1 balun .
BTW , the 40 m dipole is mounted about 8 feet above the KT34 BUT with its
element in line with the KT34 boom !!!!!!!!!
Thanks to K7ZD who spent hours and hours giving me the signals every 100 khz
across the bands and my Grandson who spent hours and hours sitting on roof
turning the antenna on the tripod hundreds of times . I started this fiasco
with a MFJ259 taking data - thanks to the AIM4170 which gave me the ability
to do those SWR plots in seconds instead of minuites plus being far more
accurate.and letting me do several other sorts of measurements . Also thanks
to Bill N6MW who was suffering with the same KT34 F/B on 15 and shared the
misery and did a lot of MANNA modeling and sharing.
My 15 meter trap
dimensions ended up
M2
REF 48.75" 46.75
RDE 48.5" 47.0
FDE 43.75" 44.625
DIR 43.75" 42.5
but remember these were arrived at with known lower capacitance 15 meter
trap capacitors.
I was essentially at M2 dimensions for the other bands
----- Original Message -----
From: "John Donahue" <donahue5617@roadrunner.com>
To: <towertalk@contesting.com>
Sent: Thursday, August 02, 2012 7:47 AM
Subject: [TowerTalk] KT34A to KT34M2 upgrade
I’m having the same problem high vswr on 15 and 10 meters, 20 meters was ok.
I tried increasing the distance between the shorting bars for 15 and 10
meters, the vswr then changed to better than 2 to 1 on 20 meters, it was
about 1.2 to 1 on 15 meters and over 3 to 1 on 10 meters. Found that the
vswr on 10 meters dropped to less than 1.5 to 1 but that was on 29.5mhz.
I have been talking was M Squared about this and all they keep telling me is
to check the dimensions, which I know are correct.
John Donahue, N1JD
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