--- On Sun, 5/24/09, agrimm0034 <[email protected]> wrote:
> From: agrimm0034 <[email protected]>
> Subject: [Repeater-Builder] Re: New Transmitting Antenna SWR Question
> To: [email protected]
> Date: Sunday, May 24, 2009, 11:36 AM
> It came with some cutting charts to
> cut the 3 inside elements inside of a plastic tube. I cut
> those and checked the SWR with 10 ft piece of RG-58 25 watts
> input and it read 1.5-1. Assumed everything was okay. Cable
> was put on a dummy load and cable tested great with no
> shortages and very low loss. 3-4 watts from 40 watt input.
> After testing several times the more power that was put into
> the feedline the higher the SWR was. The lower power I put
> in the lower the SWR was. My tower that it is on is not
> grounded and the antenna calls for DC ground for lightning
> protection so I might need to ground the antenna. Anyone
> think that will help SWR any?
>
>
I have not been following the whole thread,but it seems that something is wrong
with your meter. I guess that you are using about 50 feet or more of the
lmr400. It has a loss of over 2 db (almost 3 db) per 100 feet. That is a loss
of 1 db for 50 feet which should give you around a 10 watt loss or more with 40
watts input.
Many inexpensive meters (and some expensive ones) will show lower swr at lower
power. This is a false reading due to the way the detecting diodes work. What
kind of meter are you using ?
When you used the rg58 to test the antenna, it has too much loss at 450 mhz to
make a good swr measurment. I think if it shows the swr to be 1.5 it will
really be over 2 at the antenna due to the loss in the coax.
Grounding a 40 foot will have no effect on the swr of an antenna mounted to it.
Atleast if it is anywhere near the top. It should be grounded for lightning
protection if possiable.
Is the antenna at the very top of the tower or is it side mounted ? If side
mounted, this could have some effect on the swr.
