[Microwave] testing waveguide coax transitions

[Dr. Gerald N. Johnson]

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.

Some of the WR75 transitions I have acquired came with a couple tuning 
screws. Since most of our use is very narrow band, even bad mismatches 
can be corrected with tuning screws without imposing a bandwidth 
limitation. Those I have used for experiments so far like taking a feed 
horn to MUD or CSVHF for antenna range testing and I have just used a 
directional coupler as a load bridge (in coax or waveguide depending on 
the part being tested) and with a power meter of the return power or a 
detector and HP415E SWR meter and a 1kHz modulated signal source, tuned 
the screws for minimum reflected power, or greatest return loss (same 
result different names). The directivity of the directional coupler can 
affect the final result to some extent but for ham purposes most are 
good enough.

On those transitions the screws appear to be about #4-40 (about .110" 
diameter or 2.7mm) diameter and are spaced 1/2" and 3/4" from the probe. 
They have Farinon part numbers.

A WR90 load would probably match well, especially if there was a tapered 
guide 3 or more wavelengths long between the WR90 and the WR75 port.

i want to do some experiments butting WR62 to WR90 and have the same 
load quality problem, I have the parts to build a WR62 slotted line so I 
can check the load quality at 10.368 GHz in WR62 and then change to my 
WR90 slotted line to use the old fashioned vector circuit analyzer to 
look at the effects of the butted connection and the alignment of the 
guides, centered or corners aligned or faces aligned to see if any is 
workable or better than the other possibilities. I learned using slotted 
lines a long time ago and they still work, just don't make 1000 
measurements per second over a 7 GHz frequency span like a modern vector 
network analyzer but the good modern ones cost as much as a house in the 
country and I'd rather have the house.

73, Jerry, K0CQ

On 10/31/2016 4:35 AM, Dave Brown wrote:
> I have several w/g coax transitions (WR75, in this case)  that I want to
> test for possible use on 10 GHz. They are ex 13GHz equipment.
> As I see it, I need a well matched load on the w/g port to sweep the
> transition, while looking into the coax port. I have to test them this
> way as I dont have anything I could use to test them looking into the
> w/g port. Question is- what should I use for the low SWR load on the w/g
> port? To properly characterise the transition alone, a known low SWR
> load in WR75 (at 10.3 GHz) is what I need.
> I have one or two w/g loads in WR75 that I could use but they are ex 13
> GHz equipment and I dont know if they would have low SWR at 10.3 GHz.
> But I need a transition with known characteristics at 10.3 GHz to check
> the w/g loads....This seems like a chicken and egg situation! Similarly,
> a small horn with WR75 feed might have good enough return loss to be
> used as the load on the transition, but again, its a matter of getting a
> known transition in WR75 to check the horn first! Suggestions etc welcomed.
> 73
> Dave, ZL3FJ
>
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