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Author Topic: Tower Mounted 5BTV  (Read 2071 times)

Posts: 1

« on: February 18, 2012, 11:05:59 AM »

Anyone have an idea of how well a 5BTV five band trap vertical would work if mounted to the top of a well grounded 30-35 foot tower?

TNX & 73

Bill W4FSV

Posts: 12629

« Reply #1 on: February 18, 2012, 04:42:38 PM »

It will need at least a few radials for a counterpoise. They can be sloping wires.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 60

« Reply #2 on: February 18, 2012, 05:22:27 PM »

Mounting the vertical at that height mitigates both ground return losses as well as absorption losses from nearby objects.  It will be a good performer as long as you have four, or (preferably) more radials for 40m. Three radials/band for the upper bands should be sufficient

Posts: 270


« Reply #3 on: February 26, 2012, 09:39:50 AM »

Believe it or not, my roof mounted elevated 6BTV has only one radial each for 80 and 40 and performs well on those bands. I have 2 to 4 radials for each of the other bands.

I keep the antenna electrically isolated from the mast and support assembly. I use 1-1/4" PVC for this purpose.  IIRC the Hustler manual says not to ground the mast as it will tend to detune the antenna. I would not connect the tower to the radial system at the antenna feedpoint.

IMHO you will get the best performance out of this antenna with a good set of tuned radials. Some trial and error is necessary to achieve this. 

As I see it, a potential problem with a tower mounted installation is that you will not have anywhere convenient to tie off the shorter radials needed for the higher bands, and you will end up with a lot of long ropes tied to the ends of those shorter radials.

I don't think a tower (even well-grounded) under a BTV will make an effective counterpoise.

Posts: 7718

« Reply #4 on: February 26, 2012, 11:50:49 AM »

Unless the 5BTV is decoupled from the tower you will have an offset fed vertical working against ground. The tower becomes part of the vertical. This can work but the input impedance will be odd on some bands as well as exhibiting an odd elevation radiation pattern. Ground mounted radials will be required. I can model this if you like. For example, on the 10 meter band the antenna becomes a 39' vertical fed 20% from the top end. The input impedance with a good ground (at the tower base) is 50 - j500 ohms which is a VSWR of 100:1. On the 40 meter band the input impedance is 450 -j450 ohms or a VSWR of 18:1.

1) How to decouple The tower? The first thing is to insulate the 5BTV "ground" from the tower mast. This can be done with common PVC water pipe.

2) The coax shield needs to be decoupled with a 1:1 current choke (balun).

3) Two radials per band mounted in opposite directions need to be installed to form a non-radiating counterpoise. That makes for 10 radial wires.

So why tower mount a 5BTV? If the 5BTV is reasonably in the clear when ground mounted that is the place to put it. twenty five, 20' radials and you're more than good to go. I like to use #14 insulated THHN wire from the hardware store for this. If installed on a lawn the biodegradable plastic lawn staples do a great job of holding the radials. One staple every four feet works well. In a few months the radials disappear under the grass.

« Last Edit: February 26, 2012, 11:56:14 AM by WX7G » Logged
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