Dear Fellow Topbanders:
After miserable performance with my shunt 76 foot Rohn 25G (like
calling a 599 RA6AX for an hour and a half without a contact) I sent an
e-mail to this reflector about my amazement with the enhanced
performance by just adding the 26 foot aluminum boom on the top of the
tower. He is a short summary of the results of various changes: (Thanks
to the many amateurs who offered suggestions and tips and especially
Earl, K6SE who did some extensive "what if" modeling as well as Dave,
WX7G who sent me a 3-D current distribution model of the tower and boom.
The shunt feed is #6 stranded spaced at 18 inches and tapped at the 35
foot level. Guys are insulated from the tower.
1) As mentioned before the addition of just a 26 foot horizontal boom to
the top of the tower turned the antenna from non player to a player.
The need for an L network to match the shunt wire disappeared just as
Earl predicted from his model. With just the added boom alone I was
able to get a 1:1 VSWR at 1.890 kHz with just the series vacuum
variable. The antenna became to perform on the air as well. No more
endless calling of strong Europeans. The suggestion by some that this
was due to conditions is not the way I see it. Now I appeared to be a
first layer station working most everyone in Europe on the first call
and receiving 599, 589, 579 reports. This was not how it was for a
month prior but has been the case since the top loading changes were
made. The tower alone was fair to NA but essentially useless to Europe.
2) As the 204BA elements (Hy-Gain 20 meter 4 element monobander) were
added the resonant frequency appeared to drop accordingly. with just
three elements added (outside director, reflector and driven element)
the apparent low VSWR point dropped to 1.860
3) adding 4 130 foot #14 insulated radials to the existing system caused
the VSWR to rise slightly probably due to lowering the amount of series
ground loss resistance.
4) adding the inside director to the 204BA and sliding the beam up two
feet on the mast to the 78 foot level cause the VSWR dip, without
retuning the vacuum variable, to exist at 1.830Khz.
5) The usable bandwidth has increase from 20Khz (with the L-Network) to
about 125 Khz with the straight series feed.
6) The performance of this antenna is now very positive. Bob, G4VGO gave
me a no kidding 599 plus 20 dbs this week! I have worked RA6AX on the
first call every night now and before the top loading I could not get
him at all. (He came back to me as K1FZ once and called me Bruce and
the QSL is on the way!)
7) If a beam is not available for top loading I would strongly urge the
top set of guys to be attached to the tower and insulated about 35 feet
down the wire to provided some top loading. An inverted L, which is a
standard approach was not favored because of the pattern distortion and
high angle waste. if loading wires are used they should be exactly the
same length with the tower supporting like a T. If three or four equal
length wires are used for this purpose, like spokes in a wheel, their
overall length can be reduced somewhat.
8) The real world performance started to agree with Earl's model when I
provided him with greater details. He even took into consideration
tower rungs. I spared him having to factor in his NEC calculation for
analysis an 80 meter inverted bottom fed Delta loop hung by a rope from
the same tower. When I lowered the Delta loop to the ground the low
point VSWR went up about 8 to 10 Khz. So even proximity slopers and
dipoles can change the apparent tower resonance. They can, I believe
also reradiate or provide some cancellation of preferred radiation.
9) I had not set up a field strength meter as I should have. However
there is one thing I can tell you and that is my East running 500 foot
Beverage now shuts down the RX protection circuit on the Icom when I am
transmitting. I think that helps substantiate the fact that their is
more current along the ground than before, right?
Thanks again to all those who helped with their suggestions and adivce
in this antenna project. (KB5NJB, ZL3IX, WX7G, AJ1H, AA1K, WI5A, W8WEJ,
VE3CSK, K1XT, and of course Earl K6SE whose modeling took out a lot of
guessing and tower climbing. I think I am in the ballpark now closer to
home plate and not way out in left field with this antenna.
73
Herb, KV4FZ
St. Croix, VI
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