[Microwave] testing waveguide coax transitions

[Dr. Gerald N. Johnson]

On 10/31/2016 9:11 PM, Steve wrote:
> At 03:50 PM 10/31/2016, Dr. Gerald N. Johnson wrote:
>> It is hard to predict how a WR75 transition and load made specifically
>> for 13 GHz will work at 10 GHz though 10 GHz is in the standard
>> working range of WR75. The match depends so much on the probe location
>> and dimensions, and its easier to make it narrow band that full
>> waveguide band.
>
> You could bolt several of the WR75 transitions together then sweep them
> through 10 GHz. If they show relatively low loss, you could assume they
> most likely have fairly low loss... at least, lower than, say, -6 dB at
> 10 GHz. Personally, I would not trust a transition showing more than
> perhaps 0.3 dB loss (individually) at 10 GHz. In fact, I would not trust
> a WR-75 transition at 10 GHz without full characterization. I would
> assume it'd present a few dB, at the minimum, additional loss down at 10
> GHz, and I'd plan accordingly.

A curve here in a Mcrowave Journal handbook and buyer's guide from 1966 
shows WR90 oxygen free copper wave guide has 3.4 dB loss per 100 feet at 
10.0 GHz. WR75 it shows at 5.2 dB loss per 100 feet. .034 and .052 per 
foot, or about .003 and .0043 dB per inch. Not much waveguide loss in 
the typical inch or two inch long transition. The big question is how 
the probe is tuned whether its a waveguide bandwidth probe or unique to 
13 GHz. The recommended frequency range for WR75 is 10 to 15 GHz and the 
cut off frequency is 7.868 GHz so 10 is fairly far from cutoff. Tuning 
screws won't introduce much loss and could help the match and the loss.
>
> Instead, I would look for WR-90 transitions, which are rated 8 - 12 GHz
> and perform as such. Here in the States, such are easy to find, but in
> New Zealand, I suppose they're much more difficult to find and acquire.
> Transfer of Technology issues may apply, I dunno.

WR90 has been in use a very long time but it has potential mode problems 
at 13 GHz as used for satellite uplinks, hence the popular use of WR75. 
WR75 hardware like transitions, feed horns, LNBs, and switches is easier 
to find today than WR90 though I have some of both. I'm thinking of 
mixing them to use pieces of a 12 GHz LNB retuned as a 10.4 GHz LNA with 
WR75 waveguide while using WR90 on the transmite and antenna sides and a 
WR90 switch for the TR relay. So far my coaxial LNAs have not proven to 
work well. Most of the 11 to 13 GHz satellite hardware is WR75 and its 
known to work decently at 10 GHz.

73, Jerry, K0CQ
>
> SteveH
>
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