I began using insulated elements 25 years ago as a result of a very
specific experience that I'll describe below. Although the situation has
changed (I use monoband Yagis on separate towers now), I've retained the
insulated element construction practice. Here's the story:
Just prior to the 1973 CQ WW CW DX Contest I mounted an all-metal (no
insulated elements) 20 meter Yagi on a 24 foot boom about 10 feet above my
all-metal gamma match fed 3 element 40M Yagi. I checked out the 20M Yagi
just before the contest and it was working great! 0000Z came along, 40M
was red hot, I jumped onto the bottom of the band and --- NOTHING! No one
would answer my CQs and I had some difficulty even when answering CQs.
After a frustrating hour, I went up the tower with a flashlight --
everything looked perfect! After I came back down, I checked the VSWR
compared to my records and the entire VSWR curve had shifted and it had a
radically different shape! More checks revealed that the 3 element
40M Yagi had no discernable pattern -- it was omni-directional!
Another trip up the tower revelaed nothing out of the ordinary... just
that new 20M Yagi...
After some thought I decided that the problem had to be the new antenna.
The 3 element 40M Yagi had always been an incredibly good antenna, and the
only change was the new 20M Yagi.
I decided to fabricate an insulated mounting bracket for the 20M boom and
install it during the day Saturday, isolating the 20M boom from the mast.
I installed it on Saturday afternoon after the European runs had slowed
down and -- voila! the problem was solved!!! Apparently the 3 element
20M all metal Yagi was top loading the 10 foot mast, creating a nice 40M
1/4 vertical. The all-metal gamma-matched driven element was feeding lots
of power into that 1/4 wave vertical. Breaking up the "1/4 wave vertical"
with an insulator solved the problem.
In the weeks after the DX contest, I replaced all off the element-to-boom
plates on all of my Yagis with "Type LE" linen plates. This is an
extremely strong material virtually impervious to UV and other weather
related deterioration. 25 years later, I'm still using those Type LE
plates and they are faded but otherwise like new. All of my Yagis still
use insultaed elements (as does KLM and most of the high performance K2RIW
style VHF/UHF antennas).
Good luck on ur plans, we'll discuss them in detail at Dayton!
On Sat, 10 May 1997, Michael Wetzel wrote:
> Hi Frank,
> I've enjoyed and learned a lot from your comments on your antenna
> construction techniques, etc. One thing that I (missed) haven't seen has
> been your reasons for using insulated elements. Can you shed some light on
> I am in the process of redoing my 20 and 40 meter antennas and also
> increasing the height of the tower that they are on. Previously I've had 7
> element log-yagis at 75' and 150' with a 3 element 40 at 140' (booms were
> 52' on 20 and 42' on 40). I am probably going to raise the tower to 160'
> and go with 46' booms on 20 m at about 55', 105' and 170' with 40 m planned
> for 160' and 80'. With my guys points I am limited to the 46 or maybe 48'
> boom on the lower 20's and be able to rotate. Any comments or suggestions?
> I kept seeing that 200 ohms in your files and couldn't figure it out until
> your comments on the t match!
> Mike W9RE
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com