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The RF SIDE - par deux The Repeater saga.

repeater Low Pass Filter

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#1 wqzw301



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Posted 10 January 2020 - 04:49 PM

So this is what I got, on the rf side of things...


Motorola Radius M1225 UHF 20-40watt

ID o Matic controller

RG-214 jumpers with N (non magnetic - nickel free) connectors

LNA from Downeast Microwave GaAs Phempt >17db gain 0.5db noise figure.

Harmonic Preselector 3 pole - 1.2 db loss - pulled from a Motorola MTR2000 brick

Vertex 6 cavity XUFIP0001 notch duplxer . 40watts in 37watts out on my meter.

50ft of DX Engineering 400max to a slim jim 3/8" copper - mounted on a 40ft fiberglass mast - away from all things metal...


My question

Should I add a Low Pass Filter? I kinda want to....

I've used them before on vhf transceiver's and have had great success in the cell tower jungle of Brooklyn....

BUT...Is there a point where more becomes less.....in line to the receiver

My neighborhood did just get new 800mhz cell towers in the area. About 1/4 mile away....

I have a LPF pulled from a GE Harrison repeater (432 - 470mhz). Can't find specs for it. Don't know the db loss...

I am looking forward to experimenting.....but due to my situation, I can not run around with an ht right now...

Plus I have learned on my own and would like to hear from people who actually know what they are doing and can add to my knowledge...



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#2 RF Medic

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Posted 05 February 2020 - 12:03 AM

Adding the LPF is a waste. The LPF would be necessary if you were running a circulator on the output of the transmit radio to supress the harmonics generated by the magnetic core.

Put more effort into your antenna system. Heliax and a better antenna. Decibel DB404/408/420 or a Commander PD-201 or PD-455. Thats the weak part of your system from what you have described.

Finally browsing the forum. My day to day rf engineering workload here in Gotham has kept me too busy for the hobbyist side of things.


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#3 WPXM352


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Posted 05 February 2020 - 02:00 PM

I would be wary of that LNA. Assuming your duplexing and preselector is working properly to reject TX noise and carrier (TNRD) to the degree that the additional low noise is useable, i would install an attenuator after that LNA to remove excess gain before the receiver. You do have that preselector between the duplexer and LNA I assume?

Have you performed a duplex sensitivity (effective sensitivit) test into dummy load and antenna?

If you don't run this basic test, the repeater can be functionally deaf.
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