[Elecraft] RX ANT PORT

[Al Lorona]

This is a message I prepared a while ago but never sent. Now that Mike K4PI has 
brought up the subject I thought this might be of interest without increasing 
the traffic too much.
 
We don't often pay attention to SWR in a receiving situation. If you have an 
antenna analyzer you can measure the SWR looking into your receiver; I measured 
the SWR of my receiver input as 1.5 (best case) and 2.1 (worst case) in the 40 
meter band. This may surprise some of you, but I have found these to be very 
typical numbers for ham receivers.
 
That's looking into the receiver. Looking the other way, into the antenna -- or 
in my case the antenna tuner-- I find that the best SWR I can reliably get by 
adjusting the tuner is 1.05.
 
This is a classic mismatch situation. Two 'blocks'--  in this case the receiver 
input and the antenna tuner input-- that are not perfect 50 ohm impedances are 
connected together. There will be reflections between the two stages that cause 
'mismatch loss'. Mismatch loss is the loss of signal due to imperfect 
terminations.

It is easy to calculate how much loss mismatches in a system will cause. The 
equations are not difficult [ http://en.wikipedia.org/wiki/Mismatch_loss ] but I 
use my Hewlett-Packard Mismatch Calculator (which is two sliding pieces of 
cardboard: old school, pre-smart phone apps!) In the example above, between the 
2.1 and 1.05 SWRs, it turns out that the loss is about 0.8 dB. This means that 
the signal-to-noise ratio of the signal you're receiving will degrade by 0.8 dB.
 
Let's let the receiver get worse; let's say that it is a 2.5-to-1 mismatch, 
which means that its input impedance is 125 (or 20) rather than 50 ohms. In this 
case, the mismatch loss becomes 1.2 dB which in the vast majority of situations 
is undetectable.
 
Now imagine that our receiver is particularly good and has an input SWR of 1.5 
to 1. The mismatch loss is only 0.28 dB worst case, which is pretty much 
unhearable.
 
You may be asking, "What if the antenna match is worse than the 1.05-to-1 used 
in this example?", and that's a great question. In the case where the 
antenna match increases to 1.5-to-1 and the receiver is also 1.5-to-1 the 
mismatch loss is definitely worse, but still less than 0.9 dB worst case.
 
In most cases for the typical mismatches found in our HF systems a good 
rule-of-thumb is to assume the receive mismatch losses are on the order of 1 dB.
 
Nothing in electronics is perfect, but fortunately when it comes to the 
imperfect input impedances of our receivers, we don't pay too high a price for 
the imperfection.
 
Al  W6LX




>>Has anyone done a check on the input impedance of the receive antenna port?  My 
>>crude check showed around 33 ohms.  Just curious.  73 Mike K4PI