On Monday 15 September, I asked for some advice and opinions using the KLM
KT-34xa tribander to top-load my 84ft tower for shunt-feeding on 160. I
specifically asked if it was important, worthwhile or advisable to
connect electrically the refelector and last director of this beam to
ensure that I got full advantage of the possible top-loading.
The majority opinion is that the elements of the KT34xa "as is" will not
contrubute much to top-loading the tower.
Opinion is divided on whether the performance of the KT-34xa would be harmed
I connected the reflector and third director to the boom, although W4ZV
reports convincingly that his '34 perfomed well when he connected all the
parasitic elements to the boom.
Thanks for all the responses and helpful advice. What a great hobby this
is! Below, you'll see some of the responses I received.
73, Dave VE2ZP/VE9CB
ve2zp@bbs.ve3jf.ampr.org
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>From Rod VE7FPT:
I have a kt34xa on a 64 ft tower, shunt fed on 160. The elements are
insulated. My modelling with elnec suggests that the antenna is
contributing more toploading that just what the boom would provide.
In fact, modelling with the boom and the two end elements connected
to the boom seems to work out about right, but there could be many
other factors affecting this. I have wondered what would happen if
the elements were electrically connected to the boom. I hope someone
with the answer will answer your posting.
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>From Gerard F6FGZ:
I use a 70 ft tower as shunt fed tower for 160. It has at top a
KT34XA + 40-2CD by Cushcraft. I let both antennas unmodified, so
I can't answer if the KT34XA alone will be sufficient for your
tower, BYW the 40-2CD has elements isolated as well. I use a
gamma match feeding made of 1/2" aluminium tubing (from an old
crashed antenna...) of 42 ft at a spacing of 3 ft. Spacing is
very critical in matching so I used PVC tubing for standing on
the tower which are easy to slide and adjust spacing from the
ground level. Capacitor is a 300 pF / 7 kV. For the moment only
7 long buried radials (120 ft) are used wtih success.
See you on Top Band.
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>From Peter VK3QI:
Before I can answer these questions the following must be clarified:
How is the KT34XA fed with the coax and how is the coax grounded to the tower?
If, as I suspect, the coax braid is grounded to the tower some where near
the base of the tower, then when the tower is shunt fed on 160mx, the WHOLE
of the coax braid becomes part of the tower system at the same potential and
consequently there is a DC (and hence RF) connection all the way up the
tower onto the KT34XA elements. Depending on the feed system (balun etc.)
it may be that every second element (alternating) is connected to the braid
side of the coax.
ON4UN is not quite right in saying the AREA of the top, rather the effective
length from the centre of the boom out to the longest element tip.
If your coax braid is entirely floating from DC then I would suggest that
you try grounding it, not only to improve on 160mx but as an obvious
lightning protection.
PROOF: I have used the following configurations on top of my tower and
notice NO change in tuning.
1. A 26 foot boom 204BA
2. A 24 foot boom tribander with grounded elements
3. A 20 foot boom tribander with 4 elements (including 7 mhz loading coils)
all insulated from the boom (with coiled coax balun, not grounded to the
boom) but grounded at base of tower and running up through centre of tower.
Good luck with your endeavours on 160 mx and I hope to work you soon!!!
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>From Jim KF4HK:
The KT-34XA has less top loading than a 3el. 203BA on 16 ft. boom.
Do Not ground elements to boom! It upsets the beam/currents.
Since it is on the ground, try to find a TH-6/TH-7 OR even an a-$S on 18 ft.
boom for top loading.
The XA is a great beam , but when I replaced 5 el 10m & 4 el 15 m beams with
an Xa, my 160 vertical went to hell !! Been there-done that.
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>From Bill W4ZV:
Dave, I installed a KT-34XA in 1983 at my previous Colorado location and
grounded all parasitic elements to the boom primarily for lightning
protection and to hopefully minimize precipitation static. The antenna
performed very well and BTW still holds the USA record for the CQWW SSB
Contest on 28 MHz set in 1988. I also grounded the outer conductor of the
coax feedline at the point it connected to the KLM balun. I have no idea
how to model this on EZNEC but would imagine you could at least assume
straight elements the physical length of the linear loaded elements. Hope
this is of use to you and others.
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John Goller K9UWA
Hi Dave...modeled your project on K6STI's AO-6....84 feet of
tower.(assumed it was Rohn 45g...18" on a side)......modeled the
KT-34XA as only 3 elements.....last director and reflector plus
driven element...all 3 of them 25 feet long..............if all
elements are grounded to boom.....resonance is 1.935 Mhz...in other
words very close to a 1/4 wave vertical.....feed impedance is down
around 23 ohms as would be expected due to short height and top
loading.....
If you insulate the 3 elements from the boom resonance now is at 2.3
Mhz....just the tower and the boom enter into the vertical
system...the other elements do not have any substantial current on
them....now the feedpoint impedance is 33 ohms...about what would be
expected....almost all vertical and very little top
loading....perfect 1/4 wave vertical feedpoint impedance would be 36
ohms......oh....your mast in this model was 2" diameter and protruded
3 feet above the top of the Yagi.......if you have a 2 mtr vertical
or other antenna above the tribander this would also have to be
modeled.....also if you tower is different than what I guessed it
will make a difference....you will need a bunch of radials...as long
as you can make them.....also the modeling programs assume for
impedance information......perfect ground.....
If you use the above illustration your gamma wire would be 36 inches
spaced out from the center of the tower...shorted to tower about 73
foot level...and series capacitor of 120 pF....gamma wire made from
#10 wire...........................hope this helps you out....
de John K9UWA
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>From John K9UWA (second message)
Hi Dave...jsut read answer from W4ZV to you about your KT34XA.
If you ground all the elements to the tower....then your gamma system
would be......shorted to tower at 34 foot point....same 36 inch
spacing from center of tower...and series capacitor is now about 308
pF....if you send me all the exact measurements for the
Kt34XA..tubing diameters....linear rods...etc...and what type of
tower you are using....is easy to calculate the correct match point
for you shunt feed system....
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>From Carl KM1H:
The insulated elements will have little or no effect as a capacity hat.
I would suggest that you contact KLM about grounding the end elements. I
did that with a 4el KLM 40M beam and it worked well but I'm not familiar
with the KT-34XA.
73...Carl KM1H
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>From Lynn, NO9Z:
I not real up on the theory, but I can give you some real life results.
This summer I loaded my 100 foot tower for 160. At the top of the mast is
a 2 el cushcraft 40 (40-2cd) about 112 feet . Below that is a kt34xa
at 102 feet. I have 7 ground rods and 8 130 radials. The tower is shunt
fed at approximately 80 feet. The shunt wire spacing is approximately 30
inches. The series capacitance is approximately 100pf. I dont know how
to measure top loading, but these dimensions should get you in the ball park.
The tower seems to transmit very we
ll.
The biggest problem I had during this project was finding the correct
tap height on the tower. I started around 40 feet and swr was beautiful
at 1.535 mhz.
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Thanks also to Walt W0CP and Alberto EEA#CR fof their interest and
observations.
73, Dave VE2ZP/VE9CB
ve2zp@bbs.ve3jf.ampr.org
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