> How significant is the impact of transmission line and antenna
> matching?  What sort of dB loss should be expected, worst case, when
> simply not taking care?  How can I measure this at 10mW?

Antenna aspects can make a vast difference.
Actual antenna has gain (or loss) wrt an isotropic (omindirectional)
antenna. Gain is achieved by shaping transmission pattern to
'illuminate' less space (or less area on a sphere around the tx. For
telemetry use the sort of antennas liable to be used have gains around
zero. More possible with dipole (about +3bB), Yagi /beam (like
traditional TV antenna) - 3dB on up. 1/4 wave "whip" is common and I
think has very mild gain over isotropic. I.m sure Wikipedia will know.
A 5.8 wip can have some useful gain over a 1/4 wave.

Mismatching the drive to the antenna is VERY easy and losing more than
half your power would be common.
Losing 90% of your power is entirely doable.

As they said - ARRL handbooks of various sorts. Evan an old ARRL
Antenna book is liable to be of good value - probably low cost in US.
www.abebooks.com always a good place to look.

Various formal measurements exist but a basic meter + antenna plus
diode may suffice. At 10 mW it's harder than usual. If you have
receivers with AGC the AGC voltage will be related to signal strength.
May not be linear.

If you have some measure of signal strength from a receiver you can
walk it in and out while transmitting and measure distance to where
signal is at some reference level.
Note that receivers close to the antenna will be in the 'near field'
and results will not be representative of longer distances.
[Jodrell Bank dish is used to talk to spacecraft. Its near-field
extends to low earth orbit. A bit hard to test new receivers out of
the rear field. ]

My recollection is that, for the keen, quite good antenna matching
measurements can be done with lecher lines and working out standing
wave patterns. This is a relatively lost art.


             Russell
--=20
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