[TowerTalk] Full 40 mts dipole on side of a tower
john at kk9a.com
john at kk9a.com
Mon Apr 2 11:07:33 EDT 2018
RG392 has an aluminum braid, could you have a back shield connection?
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Good morning Craig,
You make several good points. As I measured the dipole hanging away from
the tower, there was 100 feet of RG8X used as a feedline. It is known to
be good and I check loss often. It is my "utility" roll of cable, kept for
purposes like this, antenna testing, etc...
The SWR started at 1.1:1 at 7.0 Mhz and rose smoothly to 1.5:1 at 7.3Mhz.
Obviously, the dipole needs to be shortened. But since I operate almost
exclusively on CW and hardly ever above 7.2Mhz SSB, I decided to leave it
there. I didn't use any sort of balun for this test. I have a balun on
the tower, along with the the permanent feedline, a new piece of "buryflex"
cable. The balun is one I made. It is actually a common mode choke, RG392
wrapped six times around a Type 31 core, and then repeated on another core
about six inches later. This is all my best effort to replicate info I
find on K9YC (I think) website and the K7 fellow's website, who sells the
My analyzer is a Comet CAA-500, the original, not the newer one with LCD
I have also simulated the antenna using accurate diameters and section
lengths using the Antenna Model Software. I ran analysis at 5 feet, 25
feet, 50 feet, 75 and 100 feet. I then took the data at those heights as
I was pulling the antenna up the trolly wire. It behaved just about like
the simulation said it would. It was a great exercise for checking the
accuracy of my model against the "real world".
All in all, it has been a fun project. I built the antenna out of scrap
parts left from my old Telrex yagis. Next step is to build a pair of
Moxon 2 el 40m yagis from some more of the leftovers. This dipole was
mainly a "warmup" exercise to validate the modeling technique. With this
dipole in place and the two Moxons mounted at 145 and 75 feet above and
below it, I will have a good setup to measure gain and F/R of the Moxon
stack compared to the dipole. This will be with only the 40M antennas on
the tower. Later, when I build and mount the next antennas, I can check to
see if there is any degradation of the system.
It is a really fun project and it keeps me off the street and out of the
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