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[TowerTalk] Elevated Multiband verticals without radials (Comparison)

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Subject: [TowerTalk] Elevated Multiband verticals without radials (Comparison)
From: K8RI <>
Date: Wed, 15 Aug 2012 01:24:49 -0400
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I've used the elevated, multi band verticals as back up antennas for 
some years.  In the real world, On average I've really not been able to 
tell the difference between the AV640 and  half wave, center fed sloper 
on 40.  One time the sloper will be the best and the next time the AV640 
may be the best.

I've had several Hy-Gain AV640s and am now trying a Cushcraft R8.

BTW having a VNA like the AIM 4170 takes a lot of work out of setting 
these up and answers a lot of questions.

At first blush many think these are carbon copies, but other than the 
matching network, which by necessity, needs to be similar if not the 
same, they have some portions that are quite different.

Both antennas have good and bad points.  The R8 is much more heavily 
constructed than the AV640. The insulators are much heavier at 3 to 4 
times the thickness. OTOH both are pretty much sufficient if a bit 
anemic on power handling.  OTOH while being cranked up the two have 
almost identical arcs which is pretty extreme.

The AV640 is a combination of stubs, inductive loading, and capacity hats.

The R8 is a combination of stubs, traps, and capacity hats.

On the AV640 all bands are completely independent.

I was impressed with the AV640 as they required very little tuning to 
get all bands very close to 1.0:1 while adjusting one band will not 
affect any of the others.

The R8 is much more heavily constructed, BUT I am quite disappointed 
with the quality of the work done on the pieces and poor quality 
control.  Metal work in general is clean with no burrs. It's some what 
easier to assemble than the AV640.  Stubs on the R8 are constructed of 
3/8" Al tube with 1/4" rods used for coupling. The AV640 uses 1/4 solid 
Al rods that thread together for stubs giving it a much more finished 
appearance.  On the R8 those stubs mount on 1/4" Al plate while the 
AV640 uses stamped Al fittings.   Both take a lot of space just to put 
together a vertical.  The R8 insulators are of a heavy material about 
5/16" thick. The AV640 uses Al stampings to hold a thin Bakelite 
insulator that is fairly fragile.  Fortunately they are easy to make out 
of 1/8" or 1/4" Plexiglass or Lexan.  Lexan is tough, but it's also 
expensive while the Plexiglass is is more than sufficient and you can 
often find scraps at the local hardware store for free.

The ground strap from the matching unit to  the counterpoise was not 
soldered into one of the ring terminals on the R8. It wasn't just a cold 
solder joint, the solder was only on the braid with none on the 
terminal. You can move the braid back and fourth in the terminal.

The Top insulator for the stubs would not fit over the center tube. It 
required trimming and filing until it would fit over the tube.  It 
appears that it was drilled out with a dull cutter, or one running too 
fast that overheated the material.

The holes drilled for the brackets that hold the insulator for 3 stubs 
did not line up leaving the holes for the stubs about 30 degrees out of 

It tells you to insert one tube 8" into the one below, but it has an 
insert that will only allow it to go about 7"leading you to think you've 
made a mistake, but that's the way it goes together.

The R8 uses SS hose clamps to hold the 3/8" tubing together and those 
are easily stripped even with an X-celite screwdriver handle.

Set up:

The AV640 only took a little "tweaking" to get it where I wanted it on 
all bands. SWR was less than 1.1:1 on all bands at resonance.

The R8 ... Nothing was where it said it should be for resonance and the 
2:1 SWR points were much closer together than the AV640. IE the AV640 
was much broader on all bands. On the R8 6 meters was close at roughly 
500 KHz low for where the dimensions said it should be.

I thought possibly it might be me, as IIRC I did make a mistake one 
time... But a ham bout a mile down the road just finished helping 
another ham put together an R8.  They went through two matching networks 
and finally built their own matching network which is more patience than 
I have.

With the dimensions so close I don't understand why the band width is as 
different as it is.  About the only thing I can think of is the matching 
network as the best SWR for the R8 is 1.25:1 on 40. The same point 
(7.175) on the AV640 is 1.01:1 at it covers the entire 40 meter band at 
3:1 or less. The 40 meter band edges on the R8 are close to 5:1.

Neither of these are really legal limit antennas although the AV640 is 
advertized at 2 minutes, legal limit, key down. On the R8 "they say" 1.5 
Kw PEP on SSB but that is with little or no speech processing.  CW is 
750 watts on the R8.

My guess is the limiting factor is the matching networks used and the 
compensating cap.  That and the 640 uses more capacity hats effectively 
making the elements larger.

Now on to playing with and modifying the matching networks to try and 
make them into antennas that will handle the legal limit. Mother nature 
threw the AV640 a curve (apparently lightning) so I also have to repair 
that one.  That and I keep dropping those fragile chokes used for static 
protection.  The plans call for constructing the current choke using 3 
#31 cores and 9 turns of RG-400 and build the "roughly" 4:1 balun out of 
#10 or #12 Teflon coated silver plated wire on 3 cores for  a start.  I 
don't know what I'll have to use for a compensating cap.


Roger (K8RI)


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