Dear Doc
Given the 5BTV and similar trap verticals have reduced performance on 80m
and nil on 160m, do you have a scheme for using the whole elevated system as
the antenna on these low bands, eg a tuning/switch box in the support pole?
David
G3UNA
----- Original Message -----
From: "James Duffey" <[EMAIL PROTECTED]>
To: <elecraft@mailman.qth.net>
Cc: <[EMAIL PROTECTED]>; "James Duffey" <[EMAIL PROTECTED]>
Sent: Saturday, March 03, 2007 5:04 AM
Subject: [Elecraft] Vetical Antennas
Mike - You have already received some good advice on your 5BTV
installation. Let me add my bits of advice.
1. If you use a lot of radials in an elevated installation, they don't
need to be resonant. The purpose of the radials is to provide a return
current for the transmitter and screen the antenna from the ground (in
your case the roof and house) to reduce losses. So I suggest that you
start with 16 or more radials the height of the vertical. A conventional
ground plane antenna does not really act like a ground plane antenna
unless it is fairly high above the ground.
2. A choke balun at the antenna feedpoint, one where it exits the radial
field, and one at or near the transmitter should help keep the common
mode currents down. A good choke balun can be made by winding as many
turns of the feedline around a FT240-433 or -61 ferrite core as you can
manage. There are other solutions that will work, such as ferrite beads
or an air core balun, but the ferrite toroid is simple and effective.
3. Some commercial verticals are "DC grounded" by design with an RF choke
across the feedpoint. Check to see if the 5BTV is by using an Ohm meter
across the coax connector. If you get continuity you are probably OK. If
you can add such a choke use 2 to 3 microHenrys. If this is the case, you
are in good shape, add a lightning protector, like the Polyphaser or ICE
where the coax enters the house and install a good ground. This is a good
place to add the choke balun, put it after the lightning protector.
4. The admonition against the tuner is to prevent you from using the
antenna when it is not functioning properly or from using it on a band
for which it is not intended. Using it within the band is OK.
5.Moxon's concerns about quarter wave radials stem from two points.
a. If the quarter wave length radials are not exactly the same, then
there can be considerable disparities in the current each radial carries,
along with high angle horizontal radiation and reduced radial efficiency.
Shorter or longer radials share currently more evenly even if they are
not exactly matched in length. Longer radials can be tuned to resonance
with a series capacitor.
b. Quarter wave radials are more likely to couple to the feed line
causing common mode currents on the outside of the coax. Using a choke
balun at several points on the feedline, as indicated above, will help.
You will avoid problems in radials by increasing the number of radials
you use. This will reduce the magnitude of current in each radial and
spread the current over a greater area, both important factors in
reducing ground losses and increasing efficiency.
6. Don't expect very good performance on 80M. The antenna is short and
you are unlikely to be able to put in a good enough ground system to make
it very efficient. Still, it is better than no antenna.
7. DX Engineering has a 5BTV manual on their website that may have more
information on it than you currently have. Check it out.
Let us know how it all works out. - Dr. Megacycle KK6MC/5
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