Help with Combiner ID and tuning instructions

[FMROB]

Hello all,

I have got this 4 channel cellwave UHF combiner that I would like to place on channel. I called cellwave/RFS and they said it was relatively more broadbanded than 446-451 as we have some 452 channels, however they could not provide me with any tuning info. I know that I will need to add some dummy loads on the circulators, how do I approach tuning and testing this unit. Any info, insight or help will be greatly appreciated - may even send beer. lol

Thanks, Rob


By fmbob at 2012-07-11


By fmbob at 2012-07-11


By fmbob at 2012-07-11


By fmbob at 2012-07-11


By fmbob at 2012-07-11

[Will]

Rob,

That combiner is a cavity filter - T setup. Four circulators, and cavities/branches combined at the antenna, T, port.
You can combine wide spaced frequencies as each cavity is tuned to the transmitter frequency for that branch.

On this one there is a measurement port on each branch that monitors the reflected power. That is the cable coming out of the 'gold box' connected to the circulator's # 3 port going to the jack on the panel. The gold box is the reject load that has a measurement port.

I would use a signal generator on the transmit frequency and tune the cavity for minimum loss at the antenna, T , port.
Then with a known 50 ohm load, touch up the cavity under TX power and monitor the measurement port for minimum signal. That should be minimum return loss thru the combiner branch.

[FMROB]

Will,
Thank you for the reply. We are going to give it a whirl. What do you think the minimum channel spacing on this unit is? 150 khz or greater??

Thanks, Rob

[Will]

Channel spacing... You would have to take a look at the selectivity of one of the cavities by it self to see how close the skirt falls off. I am thinking 500 k or more.

[FMROB]

Ouch, 500 K is nasty, that may be an issue as two of our channels are 425 khz apart. How do I exactly measure the skirt fall off. Is that from center of the channel to where the skirt goes to or below zero db, hence the bandwidth from center to fall off is the selectivity?

Thanks again, Rob

[Will]

That would be where the skirt of the cavity is at the most insertion loss. So look at the loss at 425 khz away from the center frequency of the cavity.
Then tune the other cavity to the frequency 425 khz away and look how much loss is at the other freq. You do not want any interaction between the two cavities.

Kinda like two mountains close together with a very deep valley separating the two peaks. The deeper the valley the more separation and less interaction.

[FMROB]

Ok, I got it... Thanks again. I will let you know how this all turns out.