Re: [Repeater-Builder] Coupling cavities, cable length, and notch/bandpass configuration

[Kris Kirby]

On Sat, 15 May 2010, spradio2000 wrote:
> Hello, I've been designing and building my own cavities, and 
> experimenting to find out what works best. The current project is a 
> 70cm repeater, but we are planning on a 2m repeater.

> My own design uses a length between 1/4 and 1/2 wave between the 
> cavities, this looks better after tuning with the coupling loops.
> 
> Before I continue with 2m cavities I'd like to know more about 
> coupling cavities and cable lengths. What is wise, and is the setup 
> with 2x twoo notches and one bandpass the best solution. I tried 2x 
> tree notches. works nice but requires major retuning when connected to 
> the repeater.
> 
> I hope one has some good info, or knows where to find some.
> 
> Oh, all interconnecting cables are RG400 double shielded, silver and 
> PTFE cable.

Did you calculate the distance to include connectors and velocity factor 
of the coax? 1/4-wave between points is normal; it also tends to have a 
transforming effect as well that helps with matching. 

Another thought might be to aperture couple the cavities. Then you 
won't have any cable or connector issues to deal with. The paging cans 
use this to reduce losses and increase power handling capability.

--
Kris Kirby, KE4AHR
Disinformation Analyst

[Repeater-Builder] Coupling cavities, cable length, and notch/bandpass configuration

[spradio2000]

Hello, I've been designing and building my own cavities, and experimenting to 
find out what works best. The current project is a 70cm repeater, but we are 
planning on a 2m repeater.

I've been strugling with cable lengths to connect the difrent cavities.

At the moment I use a system of two notches and a bandpass in both tx and rx 
line.

I found coupling of notch cavities to work well if not 1/2 wave or increments 
(3 notches at 1/4 wave give 1/2 wave between 1 and 3).
Bandpasses coupled at 1/4 wave seems to give a wide bandpass with a dip.

I used 1/4 wave pieces to split the antenna line into rx and tx.

What I tried today looks kind off like this.

  ,.. ,. ,.. ,..> RX
 /   \   /  \   /   \   /
1/4 |_\_/_ 1/4  _\_/_  1/4  _\_/_
||   |  |   |   |   |
ANT <---||   |  |   |   |   |
|| N |  | N |   |BP |
||   |  |   |   |   |
||___|  |___|   |___|
 \
 1/4  \-. ,. ,.. ,---> TX
 \   /  \   /   \   /
 _\_/_  1/4 _\_/_  1/4  _\_/_
 |   |  |   |   |   |
 |   |  |   |   |   |
 | N |  | N |   |BP |
 |   |  |   |   |   |
 |___|  |___|   |___|

This gives me some unwanted small dips near de passband.

My own design uses a length between 1/4 and 1/2 wave between the cavities, this 
looks better after tuning with the coupling loops.

Before I continue with 2m cavities I'd like to know more about coupling 
cavities and cable lengths. What is wise, and is the setup with 2x twoo notches 
and one bandpass the best solution. I tried 2x tree notches. works nice but 
requires major retuning when connected to the repeater.

I hope one has some good info, or knows where to find some.

Oh, all interconnecting cables are RG400 double shielded, silver and PTFE cable.

Regerads,

   Edwin