[time-nuts] "Better" gps antennas than a Symmetricom 58532A

[johncroos at aol.com]

Just a few comments on the cable and VSWR business.

The short version is that unless the run is longer than 100 Ft any antenna with a preamp gain of more than 30 dB will probably do and the VSWR business does not matter at all unless it is truly terrible.

For example the loss due to mismatch for various
VSWRS is as follows

VSWR     Mismatch Loss

1.5           0.2 dB
2.0           0.5 dB
3.0           1.2 dB
4.0           1.9 dB

Source Microwave Engineer's Handbook 1968

Cable Loss is a consideration for long runs.
All data at 1.5 GHz.

Cable         Loss / 100 Ft

RG-6            14 dB
RG-8/RG313      5.8 dB

Andrew Cable - Superflex Heliax

1/4 inch Heliax  7.47 dB
3/8 inch Heliax 5.12 dB

Sources - Andrew Catalog, ITT Reference Data
for Radio Engineers

Now you have to consider the effect on the VSWR
at one end of the cable at the other end. Here we define "Return Loss" as the round trip loss of the cable if the far end is SHORTED. That is an
infinite VSWR with 100% reflected power. This is Twice the 1 - way loss.

Return loss is just another expression for VSWR and one can be converted to the other - Thus

Return Loss      Equivalent VSWR at input

28 dB (RG-6 shorted)          1.05 to 1
14.4 dB(1/4" Heliax shorted)  1.5 to 1
11.6 dB (RG-213 shorted)      1.74 to 1
10.2 dB (3/8" Heliax shorted) 2.0 to 1

So we see that with 100 FT of very good cable, even a infinite VSWR at the far end produces
a VSWR at the input end of 2 to 1 or less, resulting in a mismatch loss of 0.48 dB or less.

This is why the T-bolt and other manufacturers state that the cable impedance simply does not matter.

I did not treat the other consideration which is
overall noise figure. This is because the receiver and antenna designers use such high gain pre-amplifiers in the antenna. If you follow this up on the Web you can find that with enough preamp gain (subtracting the cable loss)the effect of the cable and the noise figure of the second stage (the receiver) is totally swamped and the first stage noise figure is  maintained. By the way in the basic noise figure equation the value for NF is an absolute number - it is not in db.

The short version is that if you have a 30 dB preamp in the antenna and 10 dB of cable loss, you are still 20 db to the good. The noise figure is degraded by (crudely) 1/20 dB or roughly 1%. Obviously more preamp gain is better as is a shorter or better cable, but the basic system design assumes use of economic cable like RG-6.


-73 john k6iql