Re: WG20 Round flange


Mark GM4ISM
 

John,
Quite possibly, good enough for amateur purposes anyway.
I had not considered this. I have certainly measured moderate losses at 24GHz with such cable assemblies.
 
a few caveats
You will probably have to get the cable you use checked out on good testgear.  microwave round tables are good for this, (or a visit to my shack.. not too convenient from GW i guess)
I suspect the attenuation and match may be affected by bending the cable so once measured it should be left well alone.
Avoid Elbow connectors!
I am not sure if the impedance of such cables is always  50Ohms to a reasonable accuracy.  If you are buying new coax for this purpose you can choose cable and connectors to the appropriate spec.
 
I have no idea as to the absolute stability of this sort of cable over time, or over a temperature range but given the large numbers of these cables in use in microwave gear, I don't expect you will see much change when in use. Kept away from dampness, in the shack at a relatively constant temperature I would imagine this would be OK 
You should be able to dissipate a reasonable amount of power in these cables. With 50W of 10GHz into a length of  141 semi-rigid, the losses are sufficient that the cable runs quite warm (I estimate I am easily dissipating a 2-5 watts in the short cable between the TWT output and the waveguide transition in my PA
If the cable gets really hot the coax insulation expands and soldered SMA connectors fall off.  I have seen this several times.
 
I have just done a few measurements on some pre-made lengths I have around. The longest has 12 dB attenuation but is showing a ripple  ( 0.75dB peak to trough) on the sweep indicative of a slight mismatch in the measurement system. Measuring its match terminated in a precision load showed why, it had only 15dB return loss.
Shorter cables tested had return losses ranging from 15 to 22dB  (For what its worth the measurement set is calibrated and has a quoted 38dB directivity)
 
all told a good well terminated length of semi-rigid , calibrated using the average loss measured over the approx freq range you would not have a huge error. If you were using such a cable to drop the output to a level suitable for a power meter, the added uncertainty may well be acceptable, it is probably no worse than a surplus directional coupler or surplus attenuator. You should be able to make reasonably repeatable measurements, which with amateur equipment is about as good as you can hope for.  Even measuring the loss to a reasonable accuracy will require access to good test equipment in the first place.
 
Lastly, you simply cannot assume the cables and coaxial components you happily use at 10GHz will be good at 24GHz  Get everything checked and use only components you know work up that high.
 
 
 
 
  Regards
Mark
GM4ISM
 
 
 
 

Sent: Saturday, January 10, 2015 5:17 PM
Subject: [ukmicrowaves] Re: WG20 Round flange
 
 

Mark

Not a solution but a question for my own education:-

Would a length of suitable semi rigid cable make a useable dissipative attenuator at 24 GHz that could be reliably calibrated? I see that some of the smaller types are quoted with very high figures  of attenuation per 100ft (300-500 dB) and are rated well beyond 24G.

John

MW1FGQ

 

 

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