TopBand: Re:Top Band Antenna Recommendations?

[Earl W Cunningham]

On 12 Dec 97 14:24:00 PST selbrede%ARIA-2.EDW@mhs.elan.af.mil (Bob
Selbrede) writes:
>
>(snip)
>
>I sure am enjoying my first year of semi-serious DX'ing on Top Band and
thus 
>feel the need to make some antenna system improvements.  I've been
kicking 
>around a few alternatives and would appreciate some "words of wisdom"
from 
>fellow Top Banders prior to charging off in any one particular
direction.
>
>Even though the "low" Inverted Vee has worked out fairly well as a
transmit 
>antenna, it seems I could make better use of the tower I have the
Inverted Vee 
>hung on.  The tower is 102' of Tri-ex T-15 (similar to Rohn 25/45) with
15' of 2" 
>mast above that.  The tower is topped off with a KLM 4 element 40M beam
at 
>103' and a Hygain 204BA at 116'.  The tower is grounded at the base, but
I did 
>insulate the guy wires from the tower when I built it.  The primary
options I've 
>been kicking around are:
>
>1.  Shunt load the tower.  My instincts, research and modeling efforts
tend to 
>point me in this direction.  I would need to somehow isolate my VHF and
UHF 
>PacketCluster Node antennas from the tower, and probably take some steps

>to ensure a better bond between tower sections and between the tower and
the 
>mast/rotator.  Is there anything special that needs to be done to
protect the 
>rotator and/or control box from the high amounts of RF flowing through
the 
>tower structure?  Also, since the 40M and 20M yagis are top loading the
tower, 
>should anything special be done with their feedlines down in the shack? 
>Seems like there could be problems if they are connected to a grounding
type 
>antenna switch?  Lastly, I would have to do something about a decent
ground 
>system for this critter.  I suppose the optimum thing to do would be to
put in 
>120 or so ground radials.  That will be very difficult to do at this
location with 
>driveways, buildings, horse corrals, etc nearby.  
>
>So how about 4-8 elevated tuned radials up about 10-15' with a similar
number 
>of ground radials?  Wasn't sure if elevated radials are doable with a
shunt 
>loaded tower?  Maybe I could run them up from the bottom of the tower
out at a 
>45 degree angle or so, then straight out horizontally to the supports. 
It seems 
>like this would be the best approach.  By the way, would aluminum
electric 
>fence wire be acceptable for ground radials?  I have a mile or so of it
on hand if 
>it's usable.
======================
Being an advocate of shunt-fed towers, Bob, I favor this method.  I'm not
familiar with "PacketCluster Node" antennas, or even why they present a
problem, so I'll pass that part by for others to comment on.

I suggest you use elevated radials because you can afford to lose the ten
feet or so of radiator with that 102-footer and all of its top-loading. 
Establish the feed point for the shunt feed above ground where the
elevated radials are tied to the tower.  I see nothing wrong with using
aluminum wire for the radials -- aluminum is 4th on the list of good
conductors following gold, silver, and copper.

As far as bonding tower sections, mast, etc., together, I suppose that
would be a good practice, but I've never done that on my shunt-fed towers
and have never experienced any intermittent 160m antenna problems.

As far as rotor problems (I have a Ham-M on one tower and a Ham-2 on the
other), I took no precautions there, either.  I recall the rotor control
box pilot lamps glowed brighter with key down on 160m, so I bypassed the
lamps with something like .01 mfd micas, which took care of it.

The only precautions I've ever taken is to route the coax and rotor
cables all the way down to the ground (taped to a tower leg inside the
tower) before they leave the vicinity of the tower.
========================

>2.  Next option, forget loading the tower and go with an elevated
Inverted-L with 
>4-8 elevated radials.  This is obviously simpler to implement, but I was
curious 
>how much of the radiated energy would be soaked up by the tower
structure 
>and shunted straight to ground!  I'm guessing this would not work out as
>effectively as loading the tower itself?  More aperture using the tower?
>
>3.  The last alternative I was considering is using several Quarter
Slopers with a 
>remote switch box at the top of the tower.  I suspect, however, that the

>performance would not be very hot as compared to full size slopers. 
Maybe 
>turn things upside down and use Quarter Slopers fed at the bottom end
using 
>elevated radials under each one?
====================
1/4-wave slopers are an effective means of getting the tower they're
attached to to radiate.  The tower itself is the radiator in this case,
and the sloper adds very little to the pattern, acting somewhat like a
tuned radial.  At a 45-degree slope, the pattern has about a 4 dB F/B. 
Because the tower is the actual radiator, that still means you need a
good radial system under the tower.

I wonder what the effect of 3 or 4 of these "tuned radials" would be --
would they circularize the pattern?  Something for you to try with your
modeling software, Bob.

As far as their gain for DX goes, I used a 1/4-wave sloper here on 80m
for about a year.  Its performance was good, but after taking it down
adding 80m shunt feeds to my towers, I feel certain the shunt feed is
superior (from pileup experience).  Of course, I have a few dB gain by
phasing the towers on 80m.
=======================
>
>I suspect that loading the tower, if done correctly, would outperform
any of the 
>options listed.  Any tips on feeding this critter would be helpful as
well.  
>Comments and/or suggestions regarding these or other alternatives that I
may 
>be overlooking would certainly be appreciated. 
>
=======================
Like you, Bob, I also suspect that option #1 would outperform the others.

73, de Earl, K6SE


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