[OZAPRS] Cavity info
Norm, VK3XCI
vk3xci at aanet.com.au
Tue Jan 19 10:04:40 EST 2010
G'day Tony
FWIW I have some very old articles on a six cavity filter, and some general
cavity filter stuff. It's a poor photocopy of a photocopy but still legible.
It's very practical and has construction instructions.
I can clean it up and email or snail mail to you. We used the cavity filter at
Mt Cambewarra in the Shoalhaven on a tower that included paging. Worked a treat.
If you want I can get to it now and send 'sarvo
73 de Norm, VK3XCI
Mildura, Australia
The Wintersun City
QF15bt.
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.
>
> 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
>>> _______________________________________________
>>> OZAPRS mailing list
>>> OZAPRS at aprs.net.au
>>> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>>>
>>>
>>>
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>>
>>
>
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