"I want to use it for 80m as well,
but avoid lossy trap arrangements."
Traps aren't necessarily lossy. Good ones have extremely low loss.
If you had horizontal space, you might do a 160m/80m inverted L with a good
trap, but it sounds like you don't have space for that (I don't either!)
"Has anybody on this list had experience with using open wire type
feeders with verticals?"
I wouldn't use a tuned feeder and tuner like this for a short vertical. The
SWR on the line is astronomical, but since your line is so short that
probably doesn't pose *too much* of a problem 50 feet of window line feeding
a 14-j660 load does lose 1.6dB or so according to VK1OD's calculator. 30
feet is just under 1dB, but by itself maybe you'd tolerate that for
convenience.
The bigger problem is likely the tuner. Even if you put it right at the
base of the antenna, 600+ ohms reactance compensation is a lot, and the
tuner topology makes the problem worse.
The MFJ-949E is a T-network tuner and if I've got it right, the input and
output capacitors are just 200pF; that means the absolute minimum capacitive
reactance you can put in series with the load is :
1/(2*Pi*1.8MHz*200 picofarads) = 442 ohms
I think in your particular case, based on fiddling with W9CF's tuner
simulator (*http://fermi.la.asu.edu/w9cf/tuner/tuner.html)*, that means you
probably can't tune up at all with lengths of ladder line that would make
sense on your lot (have you tried it?).
If you extend the window line to 80 feet (or a range around there), that
makes it tunable, but you end up with about 3dB line loss and still have 1
to 3 dB lost in the tuner depending on how optimistic you are with the coil
Q in W9CF's simulator. With Q = 200, you're looking at nearly 2dB more loss
with a total matching/feeding system loss of about 5dB. Since you're
already going to have a fair bit of loss from the ground radials, maybe this
isn't ideal ;-)
If you put the tuner right near the vertical, or using a shorter less lossy
length of ladder line, tune will fail for lack of inductance.
Short circuiting across the output capacitor could help.
- - - - - - -
This is a tough load for your tuner to handle at all, certainly tough for it
to handle efficiently, and it's a vast mismatch for ladder line. I'd
personally recommend building a matching network from scratch that goes
right at the base of the antenna. Here's how I did it:
http://www.n3ox.net/projects/sixtyvert
I've since updated the system for motor-drive tuning. I used enough loading
inductance to give me a match slightly below the band, and I use a 2000pF
vacuum variable capacitor in series with motor drive to cancel the excess
inductive reactance to go higher in the band. (I now use a seperate hairpin
coil from the base of the loading coil to ground for matching, instead of
the all-in-one-tapped coil)
It's not as efficient at the top of the band as reducing the coil turns, but
the ground loss I have swamps the coil loss anyway, and even if it didn't,
all I'm going to do up there is yak with a buddy or make contest contacts.
For 80m, your antenna might need anywhere between nothing at all (my 18m
spiderbeam pole with an almost-straight wire resonantes around 3800) and a
tiny inductance (I just need a few turns to get down to 3500kHz... it's also
on that page I referenced).
But direct coax feed and a tuner would work OK on 80m. It's just 160 where
you need to cancel that big capacitive reactance that you have some
trouble. That essentially requires a good inductor right at the feedpoint.
In fact, if you want the ultimate in simplicity, you could do a hybrid
solution. Wind a hefty inductor and use a relay to switch it in series with
the antenna on 160. This will give you something like 14 ohms resistive
which you can probably feed reasonably well with short length of coax and
your tuner. The coax loss at 1.8MHz is negligible for lengths of RG-213
that make sense on your lot when you feed a pure 15 ohm resistance, and the
tuner is more likely to be able to handle the impedances at the shack end of
the line efficiently.
73,
Dan
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