Topband: Shunt Fed tower bandwidth replies summary.

[Ken Ramirez]

Here are the replies I received to my query along with the original
question and added info. 73 Ken N4UK
 
   ***Original Question****   
              I'd like to know what you guys using a shunt fed
tower on 160 are seeing for the 1.5:1 bandwidth?

******Added information sent out afterwards****** 
In my case the elevated radials are a better
solution to ground mounted. I started off with ground mounted and was
constantly tripping over the wires. The elevated are out of the way as they
are above my head when I walk around in the antenna field. In an effort to
achieve a 1.1:1 swr  between 1830 and 1840 I pulled in the shunt wire a
little more at the middle point. It is now about 6 inches away vs. about 10
inches yesterday. The closer the middle is to the tower the higher the
resonant freq becomes.When the shunt wire was fully parallel to the tower
and 18 inches away the 1.1:1 resonant frequency was 1750. With 12 inches
pulled
in at the middle only the res.freq raised up 80 kc. Pretty interesting.
I had never tried tuning it this way. It is now Perfect after a slight
adjustment of the vacuum variable! The 1.5:1 bandwidth is now 75 KCs; an
increase of 5KC.It is 1.1:1 on the Yaesu ft1000mp swr meter from 1830 to
1845 and increases rapidly above and below this. The 2:1 swr BW is 150KC. I
just hope this has not increased the losses much by pulling the shunt wire
closer to the tower in the center. The shunt wire is around 65 feet long
and the attachment point is at the 68 foot height of a 70 foot tower. The
tower has a 3el 20m yagi on top, a 5 el 10m yagi and a 40m rotating dipole.
The feedpoint is at the 8 foot level on the tower as are all the radials. 
73 Ken n4uk

******Replies Received*******       


Hi Ken.
I get 60 KHZ 1.5:1 bandwidth.
I have 80' of Rohn 45 with 24' of mast out the top. 3 beams on top - TH7 at
81', A3WS at 92' and 402CDF at 103'.
I am feeding 15' above ground with a series cap.
I am using a #12 shunt wire spaced 2' away form the tower and tied to the
top of the tower with an aluminum arm sticking out 2'.
I have 4 1/8 wave radials elevated at 15'. Using #12 wire on a ceramic to
have the radials act like 1/4 wave length.
It seems to work fine - how well, well not sure since I have nothing else
to
compare with it.

Ken
 I recall the 2:1 bandwidth for my 180' shunt fed tower
(nothing else on it) is about 70 kHz.  This is in fairly close
agreement with Fig 9-86, page 9-73 of ON4UN's latest book.  It's
hard to say what yours should be because of tower loading with 
Yagi's, elevated radials, etc. but it appears from his graph that
70 kHz at 1.5:1 is consistent with a tower of 130 degrees electrical
length.
 
Your 1.5:1 SWR bandwidth of 70 kHz sounds very good.  The bandwidth of
your shunt-fed tower depends mainly on the electrical size of the
structure.  If electrically 1/4 wavelength, the bandwidth will be
greatest.
It also makes sense that you had to decrease the spacing and height of
the shunt wire to get the SWR to 1.1:1 because now you ave established
"ground" at the 8-foot level, thereby shortening the electrical size of
the antenna.  This is also an indication that your structure is
electrically less than 1/4 wavelength.

Ken,
I see about the same as you. I see 65kHz at the radio at the 1.5:1 points.
I am feeding it with a series Capacitor about 600 PF, with the shunt arm
connected about 65' and about 2 1/2 ft from the tower.
The tower is 130' with the top 30' a rotating mast. Lots of top loading,
(top down)
20 el 432 - 130'
1 of 2 17 LBX 2 meter beam - 120'
2 el 40 - 116'
2 of 2 17 LBX 2 meter beam - 107'
TH6DXX -101'
Don't forget that too much bandwidth "can" mean low Q and less efficiency.
I have about 120 radials, plus an extensive lighting ground system.

Hi Ken
I've had a shunt fed tower at my home for top band for several years. 
Initially, I
used ground radials (all I could put in the ground) and a gamma rod of 40',
along
with Omega matching.  The gamma rod was 1 " alum conduit one foot out from
the
tower and gave me abt 50-60 Khz 2:1 vswr bandwidth.  I eventually increased
the
bandwidth to 100 - 110 Khz by adding a second gamma rod  nearly the same
size but
mounted in a 90 degree arc from the original - both were tied together at
each end.
While JAs were limited to their 1907 - 1913 window, I was able to take a tx
signal
up there for working those PAC stns that persisted in listening only in the
JA
window.  Got me a new one or two over the years.
Although I don't often use the shunt fed tower much anymore (have a phased
array
now), I checked it just now and it's still 110 Khz broad.

Ken
The 1.5:1 bandwidth of my 160m shunt-fed tower is 120kHz.
My tower is 84ft tall, with a KT-34xa and some VHF stuff for top-loading, a
45ft shunt feed 1/2" in diameter, parallel to the tower and spaced 18" fed
through a 500pf series cap (two 1000pf caps in series) and 75-ohm coax.  I
have no radials at all.
This antenna is a real performer, notwithstanding the absence of a ground
system.  I work Europeans easily, even with only 100w.
I suspect I have very substantial ground losses, in part because I have no
real ground system, confirmed by the unusual broad-bandedness of the
antenna.  If I improved (added) the ground system, I might gain greater
efficiency, but lose bandwidth.

Ken
My tower is 70 ft with a 2 el CC 40M beam on top. It is shunt fed with a 
slant wire at about 45 degrees,attached at the 32 ft level. The feed Z is 
110 ohms so I have to use an L network. No doubt this reduces my bandwidth.

I plan to change that this fall by moving the shunt base in to about 10 ft 
which modeling suggests will result in about 50 ohms. I use 60 on ground 
radials which are 120-80 ft in length depending on obstructions on the 
house side.
My 1.5 SWR bandwidth is 1800-1890 (~90 khz), which is not much better than 
yours.
I previously used an elevated radial system on a series fed L, 60 ft 
vertical and 75 ft horizontal. The initial bandwidth on that one was only 
25 khz.
Elevated radials become part of the tuning system. You have quite a few. No
doubt the very high Q that I experienced was due to the smaller number of 
radials. My swr bandwidth improved by about 15 khz when I fanned the ends 
of each radial.
Based on what I have seen, your bandwidth is better than what I would
expect.
It is my opinion that the slant wire feed is far easier to adjust than the 
wire parallel to the tower.
My on ground radials do not require any maintenance where the elevated ones
did. I had to do repairs after almost every storm due to fallen branches 
from pine trees on my lot.



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