Tony Lelieveld wrote:
Whatever the negative aspects, I'd give it a try anyway. If your TX
can handle the 2:1 VSWR it may just work. The antenna may not be as
effective when off tune by the amount you indicated but you have the
height. 2:1 VSWR is still 88.9% of the power being transmitted and
11.1 % being reflected.
Original message:
I acquired a 2 bay, 6 meter Sinclair antenna at our main
repeater site. It was tuned for 45 Mhz and that was verified
by checking the return loss. At my 6 meter repeater output,
the return loss is about -10 db (a little over 2:1 SWR).
If 10% (-10 dB) of the power is being reflected back at the transmitter
and not the antenna, you need to consider line loss. You said the
feedline is nearly 500 feet, and at 50 MHz, 1/2" heliax (which is what I
believe you said you were using) has approximately a 0.5 dB loss per
hundred feet, or approximately 2.5 dB over the entire run. For arguments
sake, let's call it 3 dB.
If you have 100W output and 10W reflected power. Only 50W is getting to
your antenna due to the 3 dB loss. Of that, to produce a reflected power
of 10W at the transmitter means you must have 20W reflected at the
antenna. You have a 3 dB loss in each direction. The specific power
levels don't matter much; the ratios are the same. 50W output and 20W
reflected (at the antenna) is significantly higher than 2:1. I don't
have a chart handy, but around 9:1 sounds in the ballpark.
For a good SWR (1.5:1) with 100W output, you should see no more than 1W
of reflected power with the losses that you have. (50W at the antenna
with 2W reflected at the antenna)
73,
Joe, KR3P
