Trap dipole questions

[WB3JOK]

I hadn't been home for a while and discovered that my 5-band trap dipole would not load (at least with my Heath single-banders which have fixed loading). Old ARRL Antenna Handbook design (32' per leg, 40m trap, 22' outer leg). Today it was warm enough (40's and sunny) so I lowered my trap dipole (a job in itself since vines had wrapped around the pulley rope and it wouldn't come down at first!) and found out why it wasn't working any more.

a) the egg insulator at the rope end had "flipped" and the wet braided rope was contacting the end of the wire, which is a high
voltage point. Maybe not a major effect but it can't help.

b) when constructing the traps I had missed a large solder splash across two turns. It wasn't a good solder joint but it couldn't
have helped either.

c) most importantly, one trap was entirely missing the 63 pf doorknob cap! The stumps of the wires were there but no cap
anywhere. A casualty of a couple of years swinging in the wind, I suppose. THAT makes a big difference especially on 40m where the trap is supposed to be resonant I had a small 75 pf Jennings "W" series vacuum cap so I took a couple of turns off the trap and re-resonated it to around 7400, (which is in the 40m band once the 2-3 pf capacitance of the egg insulator loops is added).

It's fed with about 40 feet of 75 ohm (RG-59) coax which is supposed to be a good match, although I then connect it to the output of my HM-15 SWR meter which is a 50 ohm unit, and a 2 foot RG-58 from the Heath. So what SWR might I expect: 75/50 = 1.5:1? Or 1:1 if the antenna (supposedly 73 ohms) is matched to the 75 ohm line? 

Anyway the indicated SWR on 75m was nearly 1:1, about 1.2:1 on 40m but 2.3-2.4:1 across 20m. At first I thought this meant that the 22' ends (past the traps) are the wrong length (especially since I had cut it for a trap with 63 pf and one of them is now 75 pf). But then again it doesn't seem to get higher at one end of the band as I might expect...

After perusing my '00 ARRL Handbook I came across this interesting chart which I think explains the problem on 20m:
(right-click on "View Image" for a larger view)



As you can see, with my antenna only about 25' above the ground (TV antenna tower at one end, tall tree with pulley at the other) the feedpoint impedance on 20m is going to be too high. Does this sound like a plausible explanation? I guess the only cure is to get it up higher!

-Charles

[w3jn]

THe best cure I've found for antenna ills is to scrap whatever you have and build the archtypical 125' long flat top fed with GOOD open wire line and a good balanced tooner - either homebrew or a Junkston Matchbox.  I have a very similar situation to yours height-wise.

With the exception of the A-3 tribander I had up for a few years, I've yet to find an antenna that outperforms this on all bands.   Mine even works FB on 160 with the K1JJ designed homebrew tooner.

Few things in ham radio are so simple yet effective.  IMHO why mess with anything else?

[The Slab Bacon]

I agree with John, Screw the traps!! Put up a much of a flat top as you can, feed it with balanced line, and use a good tuna ond you'll be ok-fb! traps suck and they are lossy. Dont be afraid of ladder line and tuners!! Try it. ypu'll like it!!

[W2VW]

Amen.

[WA1GFZ]

A tuner will allow you to take advantage of expanded bands. My coax dipole in the woods is only about 200 KHz wide on 80 meters. My Vee with open wire line covers the whole band.
traps equal loss. An open wire fed antenna has gain when longer than 1/2 wave.
I would use coax wound traps if I wanted to use them. I would use multiple dipoles off a common coax if I was stuck using coax.

[W3SLK]

John said:

THe best cure I've found for antenna ills is to scrap whatever you have and build the archtypical 125' long flat top fed with GOOD open wire line and a good balanced tooner - either homebrew or a Junkston Matchbox.

The big key here is "GOOD open wire line." I wished I had taken advantage of some of the 600 ohm open feeder line that I last saw at the Gaithersburg hamfest years ago. That would fit the bill. But now-a-days, you can't find it or the insulators. I know personally, I don't want to put together feed line because I'm not that skilled. The 450 ohm plastic crap is about as good as the TV twin lead. All it takes is a little moisture on the line and the SWR changes radically. I have to keep my eyes peeled for more of the old 'railroad track' line.

[WA1GFZ]

Mike the hardest part is finding or making the spreaders. The rest you can do while you watch TV. My last run was built in '83 and still in fine shape. I prefer Johnson spreaders spaced 30 inches.

[Steve - WB3HUZ]

I have some nice plastic spreaders you can have Mike. They are very light and you can snap the wire into the slots - no cutting or gluing required.

[K3ZS]

www.w7fg.net has real ladder line.   

[kc2ifr]

www.w7fg.net has real ladder line

True........but it is only 16 gage.

[WA1GFZ]

Real ladder line has Johnson cast into each spreader and the wire size is at least #12.
But the plastic spreaders are interesting.

[W2VW]

Home depot. Plexiglass sheet. Circular saw with plywood blade. Dust mask. 2 part epoxy. Drill holes just large enough to pass wire through. Use any wire size you may need based on SWR on the line.

[WA1GFZ]

Hey Dave that would work good with 3/4 inch copper heat pipe. Thin wall and light.

[W2VW]

Pump warm antifreeze through during ice storm. Small heat exchanger above the 6146 would do.