Dick,
I ran some simulations. There is very heavy coupling to the tower. That's
not such a bad thing. The tower current is 180 out-of-phase with the
inverted-L current. The GND return current is the same as with no tower, so
you don't incure higher GND losses with the tower in the scene. There is
base current between the tower and the inverted-L. For this you want a low
loss path. Connect the tower and inverted-L GND together with a wire.
Shorten the inverted-L for the best match at 1.825 MHz. It might be as high
as 4:1. If you have a tuner at the shack this will take care of things.
I recommend more radials. Several short radials will help and not use lots
of wire. Eight 25' radials would be a good start. These reduce 'current
crowding' in the earth near the inverted-L GND.
If you don't have a tuner email me and I'll see what we can come up with.
I'll be in and out today getting ready for the 'test. The plan is to operate
a 130' balloon vertical from out in the Great Salt Lake of Utah. Power will
be 5 watts.
Dave WX7G
On Thu, Dec 4, 2008 at 4:06 PM, <RLVZ@aol.com> wrote:
> Hi Guys,
>
> I'm trying to set up a 160-m. Inverted L for ARRL-160 this weekend but am
> having an interesting tuning problem. All thoughts appreciated!
>
> The Inverted L is made up of #12 stranded copper wire 136' in total length:
> 82' vertical and 54' horizontal. The vertical wire is spaced 6' from a 88'
> self-supporting tower with a Cushcraft XM-240 mounted on top. (XM-240
> elements
> are not grounded) Initial radial system is 2- 135' radials laying on the
> ground- not much but two similar radials worked pretty well on my 60' tower
> last
> year as the towers are 150' from a salt water river.
>
> Placing the Antenna Analyzer (AIM 4170) at the base of the Inverted L gets
> the following measurements:
>
> 1) With the coax braid connected to the tower base which has two 8' ground
> rods: lowest SWR is 3.9 at 1.776 Mhz and Z = 79.
>
> 2) With coax braid only connected to the two radials (and not the tower
> base): lowest SWR is 1.62 at 1.614 Mhz and Z = 81.
>
> It was my understanding that the coax braid should be connected to both the
> radials and the base of the towers gnd system, but as you can see the the
> minimum SWR and resonant freq. go crazy when I do.
>
> Any recommendations what I can do to get it working before tmw night?
>
> 73,
> Dick- K9OM
>
>
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