Topband: Inverted L Tuning Question

[Cliff Frescura]

> -----Original Message-----
> From: topband-bounces at contesting.com [mailto:topband-
> bounces at contesting.com] On Behalf Of RLVZ at aol.com
> Sent: Thursday, December 04, 2008 3:06 PM
> To: topband at contesting.com; towertalk at contesting.com
> Subject: Topband: Inverted L Tuning Question
> 
> 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


Don't bond to the tower - it increases the interaction with the antenna.
Think of the antenna as a bent vertical and add more radials.

The Z you should be looking for is around 25 ohms.  When you add more
radials and improve the ground the Z will go down.

73,

Cliff K3LL/6




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