Mobile Duplexer as a Preselector

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TylerMK

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Quick question about using a mobile "flat pack" duplexer as a modified preselector. I'm using a Yaesu DR-2X running 20 watts into a Henry 100 watt amplifier in the 70cm band. The duplexer i'm using is an older Motorola T1500 series going to about 55' of Andrew LDF-4 to an omni antenna. Everything works fairly well but I recently started exploring preselectors. When searching on ebay using the phrase "UHF preselector" several show up that look like a mobile duplexer cut in half with "in" and "out" connections. In addition to the T1500 I also have a mobile-type duplexer tuned to my frequencies. Is it possible to use only the receive side of the mobile duplexer as a type of preselector into receive side of the the T1500 duplexer? Maybe the "low" connector of the mobile duplexer to the repeater and the "antenna" connector to the receive side of the T1500? I wouldn't think that the 100 watts would be an issue since it's only on the RX side of the system. This whole thing may be a terrible idea but I figured I would throw it out there and see what information I could gather. Thanks in advance.
 

prcguy

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The flat pack mobile duplexers are notch filters and not really band pass. The do sort of band pass but its very broad with no skirts to speak of. The Chinese flat pack preselectors are window filters letting several MHz through and they are lossy with around 1.5dB or more insertion loss. They might be ok as a not so critical tower top filter before a preamp master receive antenna in an area with low RF levels. If you had a high power commercial transmitter at your same location that was specifically causing a problem a Chinese flat pack preselector tuned for your amateur receive band would reduce the level of the commercial transmitter 10 or 20Mhz away getting into your receiver.

What you need is a very narrow band filter that will help keep the transmit energy and nearby commercial frequencies out of the lousy Yaesu DR-X2 repeater receiver. I have a DR-X2 and its receiver is a challenge to work with when running any more than 5 watts on the transmitter. A single 5" cavity filter should add a good 15dB or more isolation from your transmitter 5MHz away with only about .5dB insertion loss. A bigger 10" cavity will do way better. Two 5" cavities will work really well at the expense of 1dB insertion loss and so on.

Here are some examples of filters that will make a difference.



Here is a pic of the last load of cavity filters and duplexers I had to dispose of but these were all VHF and 6m.

cavaties.JPG
 
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ramal121

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Yes, the flat pack is a notch type filter and not what you'd want as a preselector. Fear not. Since you already have a spare, you can flip the response and make it into a pass type filter. Just stick a tee on the receive port of the duplexer and then run the receive coax through the tee. This will create a high impedence at the notch freq and much lower impedence outside this which will suck up outlying freqs. Kind of like a half wave open stub filter. You'll have to sweep it to see how it actually works as there are variables that will affect the response. Have done this when I need a quickie haywire filter for test equipment. You can turn a pass can to a notch this way too.
 

TylerMK

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Thank you all for the replies. You've educated me and given me a few ideas on how to proceed. I'll try the tee method first and see how it works out since it will be the cheapest. I was actually looking at those same band pass filters on ebay, which is really what got me thinking about using the mobile duplexer in the first place. Originally I was using a GR1225 but since I migrated to the DR-2X sensitivity and selectivity has somewhat diminished. Thanks again.
 

TylerMK

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Update;

Sorry to resurrect an old thread but I finally managed to make it to the repeater site and do some "tinkering". In the end I placed a "tee" on the RX side of the mobile duplexer with a cable going to the RX side of the T1500 duplexer and cable from the opposite side of the "tee" going to the TX side of the same mobile duplexer. I then connected the RX side of the repeater to the antenna output of the mobile duplexer. This provided positive results. It should be noted that I have no real test equipment and all testing was done with the help of repeater users in the field. It's a very unconventional setup but the RX side of the repeater had definitely improved.
 

Project25_MASTR

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I'm not an expert, but it seems to me that the Motorola T1500 likely already provides more RX isolation than the mobile duplexer is capable of. Not sure if the combination of the two would be additive or not.

You'd actually be quite surpirsed. The T1500s were great duplexers once upon a time. Unfortunately, their heyday was back when repeater receivers had 2 MHz or less of band pass. The repeaters used in modern receivers are as wide as a hanger door which is why the industry has more or less migrated to 6 cavity band pass band reject designs (or four cavity with preselector).
 
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