Batboard

Hi there guys this is the challenge, I got an analog MSF5000 and the TX filters are missing. I'm planning to use two bandpass-reject filter at the amplifier output these filters have 100 dbm of isolation will this arrangement be enough?. I beleave the original filter just have 57 dbm. after the filter I will use a quarter wave jumper to a regular pl259 "T" and the same for the receiver. Any comments???

Luis

The internal UHF duplexer/filter (if that's what you're referring to) has two sections: three tuned stages before the PA, and four after the PA. The cable that feeds the antenna connection and the receiver varies depending on whether your transmit frequency is higher or lower than your receive frequency. It does play an important part in keeping transmit power out of the receiver. If I recall, the length from the output of the filter is 1/4 wavelength with TX lower than RX, and 3/4 wavelength with TX higher than RX. The cable from the TEE to the receiver doesn't seem to matter, but I think it's several feet at least, due to the RF tray needing to slide in and out.

Now, the six tuned stages in the UHF receiver each provide 15 dB rejection of a signal that's 5 MHz away from what the receiver is tuned to. Since the duplexer/filter is essentially three or four more of those same tuned circuits, I'd be willing to bet that they provide similar attenuation. However, since three are in front of the PA and four are after it, it is possible that the PA itself could generate enough energy on the receiver's frequency that only the four section "post-filter" will actully have any effect. In this case, you might only get about 60 dB of rejection of the transmitter's signal at the input to the receiver. Is this enough? Probably not. A 100 watt signal is +50dBm, and the receiver's on-frequency sensitivity is down around -127dBm. You can do the math !

I have an MSF5000 that was originally a base station. I converted it to a duplex repeater by using an external six-section Celwave BP/BR duplexer mounted to the back of the cabinet. These units are good for 300 watts and provide over 105 dB of rejection on each side, while eating up only 1-2 dB at the passband frequency. They're often seen on eBay; a realistic price is $200-$400. This would give you more than enough isolation and rejection on UHF and is what I'd recommend you try to get. Without the proper TEE cable on the output of the cans you have now, you'll be fighting with desense and poor performance for the rest of its life.

And that's my "three cents worth" !

Bob M.

My two bits worth.
We are hams:
By all means try your set up.
I would recommend that you drop the PA output to 10W or so while you experiment.
I see no reason why it would not work well.

I would not worry about the pre filter.
The MSF is designed for multi channel operation, the filters are a few meg wide to accomodate this.
In your case, you are using only one channel, so a couple of pass, or pass/ reject (which I recomend) in the PA output should be sufficent.
Your filter has much tighter skirts than the original.
Your only problem is imprdance matching, thus the critical cable lengths.

All of the MSF both analog, and digital that I work on seem to be about -117DBm/12Db sinad
I use a Motorola R2001D, or an R2600B for measurements.

I think that you could live happily with much less seperation than -100.

If your monitor has a tracking generator function you could see what effect cable length has on performance.
I use coax elbows to lengthen a cable when retuning a duplexer for a lower frequency.
This approach could be used to experiment with your cable lengths.

If after all is said and done, and you find that you have about 1.5 to 2Db
transmit insertion loss, no appreciable loss on the receive leg with no measureable desense, you have a winner.

Hi there Guys, in my life as a ham I have met many people but none like Bernard Dier. I requeted some help from him and without knowing who was I he promptly gave me the info I needed and more. I wish I could meet more people like Bernie He is a real Gentleman and a good fellow Ham. The reason I'm posting this info is because good people like Bernie need to be recognized.

N4LGG

Luis

Luis,

If you really want to use the official Moto attenuators, I mean "filters", I have several laying around. I ALWAYS remove them and use external duplexers, so have a few sets laying around. May not be quick, but if you pay the shipping I will send some to you.

Hi there guys as you know I got an old analog MSF5000. and I got it without the transmitting filters. I asked the question if I can replace them with motorola cavities and I got mixted emotions. Bernie sugested to do it and I did. I used two motorola cavities BP/BR and I tuned them to my frequencies, I used 1/2 wavelenght jumpers between the cavities and at the output to the T. I used a regular M-358 T better known as a PL259 T. The jumper from the receiver to the T is not critical I tried different jumpers and there was not change in performance. I tuned my receiver to a -119 db and it works great. The two Motorola cavities will give you about 105 db of isolation and the receiver filter have about 65 db so you get a total of 170 DB isolation between TX and RX enough for 50 watts output. That is my story.

Thanks guys.


Luis N4LGG