[TowerTalk] Inverted L SWR High

[K0FF]

Fb on your experiences Jack. Classic..... Here's what went wrong : the
original configuration had few radials and low SWR. That's because the
antenna impedance is really around 25 Ohms. With the poor radial system,the
high ground resistance (easily and other 25 Ohms)simply adds the antenna
resistance, bringing the total seen by the feedline very near 50 Ohms ( look
into "Low SWR for the Wrong Reasons" in the bible of antennas, Maxwell's
REFLECTIONS  pub. by ARRL.,,,if I understood everything in that one book
perfectly.....well maybe some day). Back to your situation, when you bought
Lance's excellent radial plate, and made a real antenna out of the "L", the
ground resistance went way down ( and the efficiency went up about 50%), BUT
the SWR went UP. The worms will curse you this winter. It'll be cold down
there in the ground again.

Hams dwell on SWR as a measure of antenna quality. Not so at all (in my
opinion).

Now for a real treat take that same antenna, and feed it at the top instead
of the bottom, and you have the K0FF upside down L. Play with the angle
against the tower but it must be steep.

My own 160 transmit antenna is an upside down L. top fed, 50 Ohm cable, no
choke, shortest possible length on the coax shield to the tower, Radiating
wire down to a post 10' high and 10' from the tower base, passing through a
big insulator the top of that pole, and kicked out horizontal at 10'
elevation to the end insulator and tied off to another post with a length of
rope. It takes longer to explain it than it does to make it and tune it.
No radials at all. No kidding.
I use the same thing on a shorter tower for 80M. (hint : the tower must be
resonant, it's the tower radiating not the wire)

Caution *extremely high voltages exists on the far end of any 1/4 wave
antenna.

Geo>K0FF
-30-

Subject: [TowerTalk] Inverted L SWR High


>
>I have an Inverted L up on 160 that runs up next to my tower for 75'.  It
is
>stood off from the tower about 3' at the top, supported by a piece of anlge
>iron and an insulator (ceramic about 4") then goes out to a tree.  The
bottom
>of the L is about 8' from the tower base.  I have a LJE radial hub that I
tie
>the radials into and have 16 radials that are about 60' long and another 16
>that are 120' long.  On the feed lne coming into the radial hub I have 50
>ferrite cores over the coax (bought as a choke kit from one of the coax
>vendors).  The problem is that I can't seem to get the SWR below 2:1.  It
>does have a nice dip at 1.830 but as I said it just won't go below a 2:1.
I
>also had an 80 meter vertical tied into the same point at the base and
>thought that it may be detuning the 160 L so I removed it.  No change.  The
>tuner on the 1000MP brings it right down but I try to make my antennas as
>resonate as possible.  The bandwidth is pretty narrow, probably about 30
kHz
>either side of the dip, where it goes to a 3:1 and then higher beyond.  I
>know some of you are going to say to shunt feed the tower but I haven't
found
>a check source of vacuum variables yet and I would like to have this one
play
>better.  It used to have a lower SWR before I put the LJE Radial Bus in and
>the metal angle standoff at the top of the tower but I can't figure how
that
>would affect anything but in a positive way.  Any comments would be
>appreciated.  Thanks and 73, Jack W4NF
>
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