Topband: 160m inv vee questions

[Dick Green WC1M]

Hi all,

 

For many years I’ve had a trapped 80m/160m inverted vee with the apex at
about 94’ on a tower that’s loaded with various yagis. The vee is oriented
so that it’s broadside to the NE and SW (wires running SE to NW). The tower
is on a steep hill, so the wire that runs to the uphill side is only about
17 feet off the ground, while the wire on the downhill side is about 27 feet
off the ground (maybe more).

 

The traps are Rayco KW-80C, which is cut for 3.625 MHz, setup for two-band
operation. On each side, the 80m portion above the trap is cut to 68 feet
and the portion below the trap is cut to ~47’, for an overall length of ~115
feet per leg. 

 

As you would expect, the bandwidth on both bands is narrow. Since I operate
almost exclusively on CW, and have an 80m delta loop with better radiation
angle and bandwidth, I only use the 80m portion of the trapped vee for an
SDR that monitors the band (due to switching limitations, the SDR can’t use
the delta loop).

 

The lower wires have been trimmed to center the antenna at 1.830 MHz on
160m. The 2:1 bandwidth is about 40 KHz, and around 70 KHz between the 3:1
marks. So the antenna is useful on most of the CW portion of the band. It
hears OK when the atmosphere is quiet, but normally I use a 520’
dual-direction NE-SW beverage for listening. As expected, the effectiveness
of the transmit portion is limited. I’ve worked at least 100 countries with
it, and in a typical contest I can work EU and SA/Caribbean if conditions
are good. But I’m usually well behind the top stations in multipliers –
maybe a little better than half what they have. Again, no surprise.

 

Recently I started thinking that maybe I should ditch the traps and convert
the antenna to a full-size 160m inverted vee. The overall length and height
of the ends above ground will be comparable. But when I compared the 160m
inverted vee to the 80m/160m trapped inverted vee in EZNEC+, there was only
marginal difference. They’re both cloud warmers at DX angles, and the SWR
bandwidths were the same. I found this somewhat surprising, given trap
losses and such. I would have expected a more noticeable difference in gain,
angle and especially bandwidth. So, my first question is, am I reading the
EZNEC+ results right, and there’s no real advantage to converting the
antenna, especially in light of losing it for SDR use on 80m?

 

Second question came up while I was reading some articles about 160m
antennas and came across one that said more radiation comes off the wires of
an inverted vee than broadside. I was under the impression that inverted
vees are omnidirectional, and if there was any directivity it would be
broadside, like a dipole. I happened to orient my trapped inv vee so it’s
broadside to EU (NE/SW) on the tiny chance there could be some directivity
in that direction. But if the article is right, or if the radiation is truly
omnidirectional, then I’m better off orienting the legs NE/SW (broadside
NW/SE) because the slope of the land would allow for the uphill leg to be
considerably higher off the ground (it would run mostly over flat ground),
though it’s not clear to me what advantage that might confer. However,
there’s a more definite advantage because the legs of the inverted vee would
be much farther away from my beverage. Right now, one leg comes within about
20 feet of it. If I reorient the antenna it would be over 100 feet away.
Comments?

 

Finally, another option would be to ditch the traps and one leg, and slope
the other leg towards EU as a ¼-wave vertical on 160m (with lots of
ground-mounted radials, of course.) Unfortunately, that would have to be the
uphill leg, so the vertical would be somewhat flatter than if I could point
it SW. Would such a vertical be superior to what I have now or the dedicated
inverted vee?

 

73, Dick WC1M