[Hallicrafters] 40 m antenna used on 80 m - what to expect?

[Mike Everette]

Well,  I once loaded up a metal bunk bed frame and springs in a college dorm room and worked some QSOs on 80 and 40 meter CW using a Globe Chief 90 and an old RCA tombstone all-wave broadcast receiver with a Navy LM freq meter as an external BFO... had a great time too, until the RF induction from the bed frame to the metal inner springs in the mattress actually started the mattress to smoldering.  (Yeah, even with that power level.)  This happened despite my use of a home brew tuning network to "match" it (the transmitter wouldn't do it alone without burning up the tank coil, tried that), and the best ground I could get through the steam radiator system.

I also worked every stereo in the dorm.  Thank goodness the RA, let alone the other residents never found out what the source of the QRM was.  Imagine NNNNN+NNT-NNNNNT-NNT  NNNNNNT-NNNNNT-NNT-NNNNNT!  coming from multiple sets of BIG speakers.  Even operating at 3 am wasn't a sure cure for this.  'Course, it was a good way to get the guy next door to quit playing acid rock at 110 dB over the threshold of pain.

And, I saw (and used) a Johnson Ranger and SX-99 setup (not mine) on Field Day once to work 10 meter AM, loaded into a barbed-wire electric fence with the wire stuck straight into the coax connector.  Both I and the guy who used the rig before me worked 'em like shootin' fish in a barrel; but 10 was wide open that Sunday afternoon.

Someone alleged once that a surplus ART-13 would "load a wet string."  A friend and I soaked quarter-inch clothesline rope in salt water, and tried it.  The rig did load it. The show of steam from the rope was impressive.  Once the water boiled out of the rope, the rig wouldn't load it any more.

But guess what... neither the bed nor the fence, nor the clothesline were fed with coax line.

Paul, I wonder if your operating awards on the wall include the WATV certificate?  (Worked All TeleVisions)  How close is your nearest neighbor?

Ever feel your hand, or arm, or whole body get a "warm" feeling when you touch the rig while it's keyed?  Ever get a tingle, or even a spark, when your hand brushes it?

Can you say, "High SWR"?

Betcha if you open up the output filter in your poor long suffering DX-60, you'll find every capacitor in it has long since been french-fried, the coils are discolored or misshapen... and if you open up that Dow-Key relay, the contacts are pitted and the tongue blued.

The best tuner for open wire or ladder line is a Johnson Match Box.  The tuner "transforms" whatever impedance the antenna exhibits at a given frequency, down to 50 ohms.  A balanced-line system does not care about SWR on the feed line; but with coax, SWR only heats up the dielectric, and your equipment.

A "4:1 Balun" seems to be the Tool-Time substitute for a tuner... but consider this: a 4:1 transformation, from 50 ohm line, gives you only 200 ohms at the output.  The impedance of a FULL wave dipole -- let's say, an 80 meter dipole loaded up on the second harmonic, 40 meters -- is over 800 ohms.  That reflects TWO HUNDRED ohms to the equipment, on the other side of the balun.  Hardly a good "match" for a 50-ohm output from a rig, is it?

A tuner is a CONTINUOUSLY-VARIABLE-RATIO balun.  

Toss the balun on your G5RV and substitute a tuner.  The difference will be dramatic.

Find yourself an older ARRL Handbook, preferably prior to 1955; or an older edition of the ARRL Antenna Book.  The newer ARRL books left out or glossed over this basic knowledge, until the return-to-favor of open wire line systems; but the newer ones (which are not edited by George Grammer, who KNEW what he was doing) still don't seem to be quite as thorough.  Or even better, get the current RSGB book "HF Antennas for All Locations" by Les Monson (expensive but WELL worth it).  

73

Mike
WA4DLF


--- On Fri, 10/3/08, Paul <w2ec at bmjsports.com> wrote:

> From: Paul <w2ec at bmjsports.com>
> Subject: RE: [Hallicrafters] 40 m antenna used on 80 m - what to expect?
> To: hallicrafters at mailman.qth.net
> Date: Friday, October 3, 2008, 6:09 PM
> Thanks for enlightening us Mike.
> 
> You know, all those years (up until 1964 when I went
> to work for Uncle Sam in strange and far off lands) we
> had no idea we couldn't use that co-ax fed 80 meter
> dipole my dad and I had set up at our summer cottage
> to work 40,20,15 and 10 meters. I guess maybe I should
> throw away all those QSL cards we received to support
> our single band WAS on both 40 and 20 meters from our
> summer cottage and all those DX cards worked on those
> bands other than 80 meters, right?
> 
> Of course proper antennas will work better, but to
> come out and say they WON'T work, as you have stated,
> is just a bit strong. Sure, our antenna didn't work to
> optimum capability and we knew it. But it worked well
> enough to get us WAS on 20 and 40 and that was only
> hamming for two full weeks during each summer, plus
> weekends whenever we could get away to the cottage.
> 
> By the way, our gear at the summer cottage was a DX-60
> fed directly to the antenna thru a Dow-key relay, no
> "tuner", and an SX-110. I'm still using the
> same rig
> today, same final tube and the tank coil hasn't melted
> yet. Of course it's only been about 45 years, so I
> don't know what you consider a LOT longer. I will
> admit many contacts were made with CW, but we did our
> fair share of AM contacts too, on all bands by the
> way. Not too shabby for an antenna that won't work on
> 40, 20 15 and 10 and will destroy our tube and melt
> our tank coil.
> 
> 73, Ray W2EC
> 
> ---------------------------------------------------
> > Oh my poor achin' pi-net... the blind are
> leadin'
> the freakin' blind.
> 
> > I feel like I'm watchin' an episode of
> "Home
> Improveement" with Tim-the-Tool-Man.
> 
> ---------------------
> 
> > An 80 meter dipole could be moderately effective on
> 30 meters, maybe 17 meters, 12 meters; but it won't
> work on 40 because that is the second harmonic.  It
> won't work on 20 either, because that's the fourth
> harmonic (an EVEN harmonic); nor will it work on 15 or
> 10 because those are also even harmonics.  
> 
> > No coax fed antenna is going to radiate well -- if
> at all --at a band lower than the one it's cut for. 
> If it ever did, it certainly didn't work 
> well!  
> 
> > Pu-LEEZE get some knowledge.  Life is a whole lot
> more fun when you are informed.  And your final
> tubeage will last a LOT longer; plus you won't melt
> the poly-insulation out of your tank coils.
> 
> 73
> 
> Mike
> WA4DLF
> 
> 
> ______________________________________________________________