TT:
Finally! This afternoon I managed to load the Trylon tower on 80M
using the method I described previously for 160M (a' la K1ZM), but with a
twist. But the BW on 80 is not as great as on 160. Here's the story.
If you will recall, I constructed four T's out of PVC pipe and inserted
them into the sides of the tower at various heights, up to 56 feet. I taped
three #14 stranded wires to the horizontal pieces of these T's, one at the
center and two close to either end of the pieces. The three wires were in
the same plane two feet off a tower face and parallel to it. I tapped the
tower (after many climbs, which I won't recount here) with the three wires
at the 51-foot level (_below_ the top support T.) I fed the bottom of the
three wires with a series cap of around 300 +/- pF and measured SWR (with
MFJ 259/731) of 1.5:1 across the entire 160M band (something must be wrong!)
(BTW - I have five #18 radial wires buried just below the ground surface and
connected to the tower legs.)
A couple of weeks ago I disconnected the center drop wire from the 160M
feed, moved it a little bit away from the center of the T, and added a
fourth wire on the opposite side of the T. I then had the two outer wires
(18 inch separation) for 160M, and the two inner wires (9 inch separation)
for 80M. All four wires were still in the same plane, two feet of the tower
face.
It didn't work. I couldn't find a tap point or cap setting that would
take the SWR below 3:1. I gave up on that arrangement.
This afternoon I attached an additional horizontal PVC crossarm to the
center support of the bottom three T's. These new arms, each a foot long,
are only 6 inches away from the tower face (the same face as the 160M
wires.) I used a couple of large cable ties to lash these additional pieces
to the T's. I removed the old 80M wires from the center of the old T's, and
attached new wires to the new arms, separating them by 9 inches (half the
separation of the 160M wires.)
After much climbing (I hope you guys appreciate this!) I ended up with
a tap point of 45 feet above ground for OK 80M matching. (The 160M tap
point is at 61 feet.) I borrowed the variable cap from the 160M feed to go
in series with the two 80M tap wires, and had to set it to near-minimum
capacitance. SWR is crappy in the low part of the band, but gets resonable
around 3.75 MHz. Resonance (min SWR of 1.3:1 or so) is around 3.88 MHz.
Now before you all go out and start climbing your towers, I haven't
tested this thing on 80M yet (still too light outside.) I plan on trying it
out this evening on a few QSOs. I also can't immediately check the
interaction with the 160M shunt feed system; I borrowed its series cap and
antenna switch position temporarily for the 80M system. But I did check the
SWR of the 160M system before I swapped the feeds - I had to tweak its
series cap a bit to restore min SWR (probably due to the presence of the two
80M wires.)
My main goal was to use only one tower face for both 80M and 160M shunt
feeding. With a little bit of additional work I think I can improve the 80M
feed's performance lower in the band (maybe tapping higher on the tower).
But for now I'm satisfied that I still have only one set of support arms to
deal with when I climb the tower.
If anyone has any additional ideas for me to improve the 80M match, I'd
appreciate hearing 'em.
73 de
Gene Smar AD3F
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