[OZAPRS] Cavity info

[Tony King]

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.

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.

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.

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

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.

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.

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.

                                                                                            Regards Tony VK3API



On 19/01/2010 8:01 AM, Ray Wells wrote:
> Tony,
>
> How do you plan to implement this;
> 1. all systems sharing a common antenna
> 2. separate antennas.
> 3. what frequencies are used for the existing system. Wider frequency
> spacing relaxes filtering requirements.
> 4  is the the existing repeater duplexed or using separate antennas
> 5. what physical spacing between antennas
> 6. what protection will the existing cavities provide both to and from
> the aprs system.It will provide some.
>
> A typical 4" cavity would be approximately 21" long, with the centre
> element being about 1" in diameter and adjustable in length to be a
> quarter wavelength at resonance. Those dimension are from an old TCA
> cavity. Typically, only the end section of the centre rod is adjustable,
> with finger stock being used to ensure reliable contact with the fixed
> portion. Being that its near the (high Z) end of the centre rod,
> currents are not high. The size of the coupling loops determines such
> things as insertion loss (and hence Q), and operating bandwidth. RFS
> cavities generally have rotateable coupling loops calibrated from 0.5dB
> to 2db insertion loss.
>
> Surface conductivity is paramount for reliable performance, particularly
> the connection between the end plate and the outer tube, with silver
> plating being commonly used on mating surfaces. Temperature stability of
> the central tuning rod is also crucial. Commercial designs usually use
> Invar because of its low temperature coefficient. The importance of this
> would be mandated by temperature stability in the hut. The RFS cavities,
> both 4" and 6" are made from aluminium tubing and the aluminium end
> plate is pressed into the tube.
>
> Many (insert more manys!) years ago AWA were producing cavities for at
> least 70-85 and 148-174MHz from copper. They were about 5" in diameter
> and were a slightly shortened design (with lower Q). They were tuned by
> capacitance. The main portion of the centre rod was fixed in length and
> a short adjustable length was fitted close to the open end of that rod.
> I never saw any performance figures on those cavities but one would
> reasonably expect performance to be worse than for a "conventional" design.
>
> Somewhere in my book collection I have an old ARRL publication, FM and
> Repeaters. IIRC there is a design in that book. If you can give me a day
> or two I'll find the book and copy some pages for you. I'll also take
> some pics of the innards of a TCA cavity.
>
> Depending on what's already on site, you may get away with something as
> simple as a single bandpass cavity, or a cavity hanging off a Tee as a
> band reject filter, in the antenna lead of the aprs system. Antenna
> isolation can be a useful tool with the isolation between two dipoles,
> one mounted directly above the other and with their ends almost
> touching, being about 30dB. Greater spacing increases that figure. Such
> things are readily measured with just a receiver and a calibrated signal
> generator. Measure the rx sensitivty. Then connect the rx to one antenna
> and the sig gen to the other. The isolation is the difference in rx
> sensitivity. With 600khz frequency separation approximately 100dB of
> isolation is required between tx and rx to avoid desense issues (depends
> of the tx and rx used). The greater the frequency separation the lower
> the isolation that is required. Isolation can come from a filter alone,
> or filters and antenna separation.
>
> Ray vk2tv
>
>
>
> Tony King wrote:
>    
>> Hi All,
>> I am currently in the process of finalising an APRS repeater for
>> coexistence with the Yarra Valley Amateur radio groups 2m repeater and
>> am at the point where I need to machine up a cavity or two.
>>
>> Can anyone point me to a suitable amateur design for a 2m unit as ex
>> commercial ones are a bit thin on the ground here at the moment.
>>
>> Actually any design formulas would also be appreciated then I could
>> design my own.
>>                                                                                          regards Tony VK3API
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>>      
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