[TowerTalk] SUMMARY OF RESPONSES TO MY 160M INVERTED L /SHUNT FED TOWER QUESTIONS

[Tom Sykes]

Thanks for the helpful suggestions regarding my 160M Inverted L questions.

The following is a summary of the responses that I received.

I.   Ron, KK1L, suggests an "80/160, Double L", as described in the
following website:

http://www.yccc.org/Features/double_l.htm

II.  Pete, KS4XG, suggests making the horizontal leg long for 175' overall
length and tuning to 50 ohms with a series capacitor. Pete uses 30 1/4wl
radials with good success.

III.  Bill, K3SV uses an inverted L which is  about 5 feet out from the Rohn
25 tower.   "The tuning capacitor at the base is in a stainless steel box,
totally waterproof.  It's been out there for several years with no water
leaks.  ....I currently use 4 tuned elevated radials that also seem to
work."

IV.  Brian, K3KO offers the following advice:

"Who in the world knows what you would end up with as far as a radiator
goes.  I'm not too keen of having variable resistance contact points from
the guys on the tower.

I hope you are going to use some other antenna for receive.
The movement of the metal guys on the tower section can generate a lot
of QRN.  It might be totally useless as a RX antenna.

You have two choices:

1.  Try it and see. When it doesn't work make sure all guys are
electrically bonded to the towers.
2.  Bond them first and then don't worry about it.

This applies to all sets of guys, not just the first."

V.  Comments from Pete, N4ZR, re: shunt feeding tower for 160M vertical:

"I  would encourage you to put a first strain insulator on each guy very
close to the tower, or (even better) to go with non-conductive guy material
from the start.  The idea is to preserve the opportunity to shunt feed the
tower on 160 later if you wish to do so, and (with non-conductive guys) to
avoid that variable in all of your antenna work.  I have almost identical
tower and antennas, and from my reading it appears that it will make a very
nice 160 vertical."

VI. Advice from Tom, N4KG:

"I once tried an inverted L next to a 130 ft tower.  It loaded great but
didn't radiate worth a darn.  Shunt feeding the tower produced a KILLER
antenna on 160.

A top loaded 95 ft tower should also be a killer."

VII.  I received the following detailed description from Gene, AD3F:

"For my Inv "L" I have the following:

1.  I placed a 1-foot diameter loop of #8 copper wire on the ground at the
base of the "L".  I soldered the ends of the loop together and kept a six
inch long piece of copper extending up out of the loop, like a rope lasso.
I drove a copper ground rod into the earth inside this loop and connected
the six inch long piece of the copper loop to it with an acorn clamp (used
with ground rods.)   I soldered some bare #12 copper (house wiring) around
the outer edge of an SO-239 coax connector, left a piece almost a foot long
extending from it, and clamped this piece into the acorn clamp along with
the ground loop wire.  Thus the coax shield (via the connector) and loop are
connected to the ground rod.  I soldered all my radials to this loop.  (The
center conductor of the coax connector is soldered to the "L" wire itself.)

2.  No experience with a tower supporting the vertical run of wire; my loop
runs up along side an oak tree.

3.  Tower ground radials must be buried deep enough to maintain electrical
conductivity with the earth during deep frost (24" here in DC area).  OTOH,
antenna radials ought to be near or on the ground surface.  The two should
not have to touch except at a common tower/"L" ground rod.  (By reading your
posting it appears as though you have only ground rods and no radial
elements as part of the tower ground.  True?)

4.  From item 1 you can see I don't use a cap; the coax connects directly to
the "L" and ground system at the SO-239.  However, I have been planning to
experiment with feeding the "L" on other bands and have looked around for
appropriate insulating, weather-resistant containers for a series cap.  Look
through a local housewares store for largish plastic containers with snap-on
lids.  Mount your cap (and DC control motor, if you choose to remotely tune
it) in the box.  I'd suggest silicone rubber sealant around the center
conductor (from the SO-239)  where it enters the box through a hole you've
punched into it.  Bring the antenna lead-in conductor into the box through
another hole on the opposite side of the box, also sealed with silicone
rubber.  Elevate the box above ground if the cap is close to the bottom of
the box.  Or mount the cap inside the box on standoffs, etc.  Don't forget a
drain hole or two to prevent condensation from building up inside the box.

     Something I've done that might help with further experimentation:  I
have two pulleys supporting my "L".  One is at the bend in the "L" (in the
branches of the oak tree) and the second pulley is in another tree at the
far end of the antenna where the haul lanyard from the end of the antenna
runs through the pulley to ground where it's tied off.  I did this so I can
raise the feedpoint of the "L" some day and experiment with raised radials.
"Simply" release the feed point at ground level (unbolt the coax connector
from the acorn clamp) and attache a rope to the coax fitting.  Then pull on
the other rope (at the other pulley) and raise the feedpoint 10 - 30 feet as
required, keeping the rope on the coax feedpoint taught.  Connect your
elevated radials to the coax fitting and run 'em out in the air."

73, Tom, NU7J






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