3 Band End Fed Half Wave with only 1 Trap. Possible?

Started by cockpitbob, November 26, 2014, 08:29:33 AM

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cockpitbob

I want 40/30/20M capability with my SOTA EFHW antenna coupler.  I want to be able to hang 1 wire from a tree and not have to worry about taking it down to change length.

I know that with one trap at the 30M point I can have a 40/30M half wave radiator.  It seems to me that since that antenna would be resonant at 40M it would also work as a 20M EFHW radiator too.  It seems like it should work, but in 2-3 years of reading about EFHW antennas I've never seen this done.

Anyone got an opinion on this?  Having one wire to string in the trees for my 40/30/20 MTR radio would help keep the complete rig package small and simple.

gil

Hello,

I think your 40m radiator wire will work on 15m, not 20.

BTW my PAR end-fed is supposed to work only for 10, 20, 40m, but it also somewhat works on 15m with a SWR close to 2:1.
They have a new model now that supposedly works for 15m as well, but not sure how different is is to mine.
You might need two traps for a 20/30/40m antenna, and it might work on 10/15m as well.

Gil.

Lamewolf

#2
Quote from: cockpitbob on November 26, 2014, 08:29:33 AM
I want 40/30/20M capability with my SOTA EFHW antenna coupler.  I want to be able to hang 1 wire from a tree and not have to worry about taking it down to change length.

I know that with one trap at the 30M point I can have a 40/30M half wave radiator.  It seems to me that since that antenna would be resonant at 40M it would also work as a 20M EFHW radiator too.  It seems like it should work, but in 2-3 years of reading about EFHW antennas I've never seen this done.

Anyone got an opinion on this?  Having one wire to string in the trees for my 40/30/20 MTR radio would help keep the complete rig package small and simple.

A 40 meter half wave wire will work well on 20 meters because it is a multiple of a half wave on 20.  I have used a 40 meter half wave on 20 many times in the past, but I do not know what effect a 30 meter trap would have on it.  I have used my EFHW tuner with multiple wires attached to it just like a fan dipole is done, so you could hang a 40 meter wire with a 30 meter wire under it and skip the trap altogether and have 20, 30, and 40 meters at the same time !

geoffrain

I have to agree with Gill,

You're using the whole wire at 40 meters, but 30 meter waves don't make it past the trap - so it looks like a 30 meter antenna to a 30 meter wave. Unfortunately the 20 meter waves won't make it past the trap either, so they see the 30 meter section of the wire.

But I've been wrong before :-)

Geoff


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cockpitbob

I devoted some more brain clock cycles to this.  It won't work with just a 30M trap.

At 7MHz the signal will go through the trap's inductor.  The inductance will add "electrical length", so the overall length of the antenna will be a little shorter than an un-trapped 40M wire.  The problem at 20M is the 14MHz signal will go through the trap's capacitor.  The cap won't add any electrical length so the overall length will be too short to be 2 half waves at 14MHz.

It looks like I need 2 traps if I want 1 wire to do all 3 bands.

Lamewolf

Quote from: cockpitbob on December 04, 2014, 01:52:20 PM
I devoted some more brain clock cycles to this.  It won't work with just a 30M trap.

At 7MHz the signal will go through the trap's inductor.  The inductance will add "electrical length", so the overall length of the antenna will be a little shorter than an un-trapped 40M wire.  The problem at 20M is the 14MHz signal will go through the trap's capacitor.  The cap won't add any electrical length so the overall length will be too short to be 2 half waves at 14MHz.

It looks like I need 2 traps if I want 1 wire to do all 3 bands.

You are correct, as the inductance in the 2 traps would combine to make it resonant on 40 meters, from the feedpoint to the fist trap would make resonant on 20 meters, and from the feedpoint to the second trap would makeup 30 meters.  Just like a 3 band trapped dipole has 2 traps in each leg.