[OZAPRS] Cavity info

[Tony King]

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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