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Author Topic: 80/40m vertical using Spiderbeam 18m fibreglass pole  (Read 2406 times)
F5VDM
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Posts: 11




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« on: December 01, 2011, 05:40:31 AM »

Hi.

I currently have a 1/4 wave vertical on 40m (basically a wire running up the side of a 10m fibre glass pole.)  It works great and I can break DX pileups wth it.  My intention is to transfer this antenna to an 18m pole, so that I can also add an 80m 1/4 antenna sharing the same ground system.

I don't want to have 2 separate antennas and wish to consolidate them into this single pole.

I was thinking of putting the 80m wire on one side of the pole to the top and the 40m on the other side to just over half way up.

Would this work?  Would there be interaction issues? I intend to feed them separately with a balun and then co-ax.

Assuming there may be interaction, how far should  I run the parallel away from the 80m wire as I could put some separators (altho prefer not to)

My goal is to keep this simple and as low cost as possible.  I don't have a junk box of variable capacitors etc, and if I don't need to make some sort of tuned circuit, I prefer not to.

This is intended as a DX antenna for 40 / 80.

Advice welcome.

thanks

Garry

F5VDM / K5XGB
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N3OX
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Posts: 8847


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« Reply #1 on: December 01, 2011, 05:58:56 AM »

It will probably work but the tuning will probably be difficult.

It's probably worth a try.

If it doesn't work, a simple coaxial cable trap could make a nice 40/80 trap vertical without requiring a well stocked junkbox.

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
F5VDM
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Posts: 11




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« Reply #2 on: December 01, 2011, 09:00:09 AM »

Further thought.

a 1/4 wave on 80m is about a 1/2 wave on 40.  Could I get away with a single wire for the 2 bands?  If so, how should I match?

thanks

Garry
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WX7G
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Posts: 6200




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« Reply #3 on: December 01, 2011, 09:22:10 AM »

Further thought.

a 1/4 wave on 80m is about a 1/2 wave on 40.  Could I get away with a single wire for the 2 bands?  If so, how should I match?

thanks

Garry

To feed a half wavelength vertical I use a 500 ohm parallel LC at the base and tap up the coil to find a 50 ohm match. For 40 meters this is 45 pF and 11.4 uH.

I ran a NEC-2 simulation of two parallel wires having lengths of 10.7 and 20 meters. They are spaced 60 cm. It is resonant at 3.5 and 7 MHz. So, the parallel wire antenna will work. It just needs tuning with a VSWR meter or better yet an antenna analyzer.
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N3OX
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« Reply #4 on: December 01, 2011, 02:58:51 PM »

a 1/4 wave on 80m is about a 1/2 wave on 40.  Could I get away with a single wire for the 2 bands?  If so, how should I match?

Matching networks for 160m through 30m that I use on my antenna are available here:

http://n3ox.net/projects/stepperswitch/matching_networks_18m.pdf

Pictures of network and switching setup are here:

http://n3ox.net/projects/stepperswitch/

My wire is 14AWG loosely wrapped 1 turn per section around the Spiderbeam pole... I do not know the exact length.

For just two bands, adapting K6MHE's 20m/40m dual bander for 40m/80m might work well:

http://k6mhe.com/files/DualBandVert.pdf

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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Posts: 13461




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« Reply #5 on: December 01, 2011, 03:24:32 PM »

Quote from: N3OX

For just two bands, adapting K6MHE's 20m/40m dual bander for 40m/80m might work well:



I came up with the same circuit years ago, only to see it published by someone else (W1FB? 
W7EL?) shortly afterwards.  The SWR bandwidth on 80m is narrower than it would be otherwise,
but the circuit works well.  I tuned it up at low power then replaced the variable capacitors with
fixed units.

Using two quarter wave elements should also work - there may be some tuning interaction,
but once you get them adjusted it shouldn't need any further attention.  You can always add
small series coils at the base to lower the resonant frequency if needed.  (Or add fixed
capacitors to raise the resonant frequency, but if you don't have capacitors you can at least
cut the wires a bit short and wind your own coils.)  This might be a good option if you can't
get them both to resonate where you want them by triming the wires.


Balun???  A common mode choke (1 : 1 current balun) in the feedline perhaps, but please not
one of those 4 : 1 baluns that are so often mis-applied for end-fed antennas.
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