[OZAPRS] Cavity info

[Ray Wells]

Hi Tony,

I was trying to upload the overnight weather to the radio station and 
drawing the remote desktop was real slow. Then the penny dropped and I 
looked at the bbs system with iptraf to see someone downloading something.

I'll still take some pics of cavities and scan a few more docs for you.

That vertical spacing alone will return lots of isolation. It's the null 
off the ends of the dipoles that do the trick and a couple of metres out 
of alignment won't do too much to upset things.

Cheers ... Ray


Tony King wrote:
> All well they are downloading as I speak
>
> All the antenna spacings are vertical with  some horizontal as well, 
> about 2 or 3 m
>
> On 20/01/2010 12:40 PM, Ray Wells wrote:
>   
>> Hi Tony,
>>
>>
>> Tony King wrote:
>>    
>>     
>>> Hi Ray and thanks for the reply,
>>>
>>> The APRS Digi will be running on a separate antenna which will be about
>>> 12 meters away from the antennas for the repeater.
>>>
>>>      
>>>       
>> Is that vertically or horizontally?
>>    
>>     
>>> As is usual for these repeater sites the hut is full of other services
>>> and even though the intermod software indicates no significant problems
>>> with interference from or to other services there is so much gear
>>> including the local FM station on the same tower I thought it would be
>>> wise to try to avoid any desense problems in advance.
>>>
>>>      
>>>       
>> Even a single cavity is a worthwhile addition on a busy site.
>>    
>>     
>>> The 2m repeater is running on two separate antennas 5m vertical
>>> separation with the receive having two band pass and one notch cavity
>>> (on the tx frequency). The Tx has a single band pass on the tx frequency.
>>>
>>>      
>>>       
>> You may find the existing cavities will provide some protection to/from
>> the aprs system. I would erect the new antenna and do some isolation
>> measurements ....
>>
>> 1. Grab a test rx and measure its sensitivity.
>> 2. feed the sig gen into the aprs antenna  and connect the rx in place
>> of the repeater rx, and again measure sensitivity. The difference is the
>> isolation.
>> 3. feed the sig gen into the aprs antenna and connect the test rx in
>> place of the repeater tx, and again measure sensitivity. Then you'll
>> have some meaningful isolation figures. Then you'll know what additional
>> measure are needed, if any.
>>    
>>     
>>> Ok on the silver plating etc. and temperature problems.  The repeater
>>> hut is air conditioned and hopefully the fairly constant temperature
>>> will mean that I don't need an Invar or similar material  compensating
>>> rod for the centre conductor, it's just that I seem to recall seeing
>>> articles on "home building" cavities way back in the early days of QST
>>> magazine and even one in A.R. and always preferring to build rather than
>>> buy reckoned it is worth a try
>>>
>>>      
>>>       
>> If you have the facilities to manufacture, why not.
>>    
>>     
>>> A google search didn't find any references and I don't' have any of the
>>> older articles.
>>>
>>> I have the machine tools to do the job and I had also thought that if
>>> design formulas were available I could adjust the width and length as
>>> required so that I could make the whole outer from one piece of
>>> aluminium bar or tube and even turn a very fine internal thread (like a
>>> camera lens thread) on the other for fine adjustment.
>>>
>>>      
>>>       
>> I'd source some large diameter tubing with a reasonable wall thickness,
>> for stability. The inner could be solid if that was more convenient.
>>
>> The RFS cavities have a course adjustment that is locked in place in a
>> threaded boss. The boss is screwed in or out for fine tuning. The TCA
>> cavities have a long threaded rod that attaches to the adjustable end
>> section of the inner conductor. Lots of ways to skin a horse.
>>    
>>     
>>> Another reason both for building and designing my own is that I also
>>> need half a dozen or so in my shack. With the APRS,  IRLP, packet BBS,
>>> and several radios on 2m all running 24/7 I suffer an awful lot of
>>> desense and cross mod.
>>>
>>>      
>>>       
>> With such close frequency spacings you may never eliminate desense but
>> every bit helps. Close frequency spacing is a pain to work with.
>>    
>>     
>>> It would also be nice if I could reduce the number of antennas hanging
>>> off my shack as well. It looks like a frightened Echidna at the moment.
>>>
>>>      
>>>       
>> You could run all receivers from a common antenna, with a bit of filter
>> protection from the tx's. Use a gasfet preamp and a splitter after the
>> filters. We used to do that on our UHF sites with up to a dozen UHF
>> repeater sharing two antennas.
>>
>> Transmitters can be combined using a variety of methods but the least
>> lossy is probably the circulator followed by a cavity for each tx. The
>> outputs of the cavites are combined as a star point for the antenna
>> connection. Fot tx only 60dB isolation is considered ok.
>>
>> I've scanned 11 pages of that book as pdf files. The files have ended up
>> a little on the large side so I've created an account for you on my
>> Linux system so you can ftp them from me.
>> ip address is 115.70.139.149
>> login vk3api (case sensitive)
>> password ynot (case sensitive)
>>
>> Yell if you have any dramas. You can do ftp transfers with Internet
>> Explorer or Mozilla.
>>
>> Cheers ... Ray
>>
>>
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>>
>>    
>>     
>
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