Posted by
James Duffey on
Mar 03, 2007; 5:04am
URL: http://elecraft.85.s1.nabble.com/Vetical-Antennas-tp444386.html
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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