[NLRS] 10 ghz dish feed

[Dr. Gerald N. Johnson]

On 8/1/2010 2:42 PM, Scott wrote:
> http://www.qth.net/k0awu/10GhzGear.html
>
> That should do it!
>
> N0EDV
>
> Dr. Gerald N. Johnson wrote:
>> I've been all over that site. I found nothing on feeds. You have a 
>> link that's not in his structure?
>>
>> 73, Jerry, K0CQ
>> ______________________________________________________________
>
>
I didn't find that page from his menu. But it sure does cover the 
modification. Which is what Ed was looking for.

In theory the thin disk is not a great short until its a quarter wave 
thick, then it acts like a non contacting short. That would be 3/4 cm or 
7.5 mm or about 9/32"  Those are round numbers for 10 GHz even. At 10368 
the thickness computes to be closer to 7.23 mm or 0.285" though the 
bandwidth like a quarter wave transformer is probably more than +/- 10% 
because the air gap open at the side opposite the signal IS a quarter 
wave transformer transforming an open to a short on the signal side of 
the slug.

It appears there are more than two ways to make the mod and have it work 
well. Were I going to tune it, I think I'd go for a fixed really solid 
short and then three tuning screws between the probe and the horn. 
Because three tuning screws at 1/8th wave spacing will just about tune 
anything. Though making the RF rattle over the threads can't help be 
increase the insertion loss over a smooth post because of the added 
surface length of the threads. A smooth post of larger diameter ought to 
have less loss, whether that can be measured on the ham test bench I 
don't know.

The three screw tuner also allows for adjusting more than just the 
distance to the short, it can tune a poorly chosed probe too, though in 
most cases the more the impedance transformation in the tuner the 
greater the tuner insertion loss. The big question as yet unanswered is 
whether the tuner loss is greater or less than the mismatch loss.

But with my dish and untuned feed measuring 32 dBi at MUD, how much can 
I gain by even lossless tuning? I suspect not much, I might gain more by 
improving the dish geometry as pointed out by W3OI last week.

73, Jerry, K0CQ
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