Impedences and Matching Question???

[centuryvrproductions]

I Have a Repeater i setup on 440, I am still learning about antennas and stuff. I was looking to use a 10db TV antenna Preamp. It is 75ohms. I have a Motorola Maxtrac 50, and i was wondering if i could use the tv amp with 50ohm coax to the reciever (Maxtrac). Or will the impedence mismatch render the whole preamp setup pointless?

 

[K9WG]

You won't see that much loss with a 75-50 ohm mismatch.

 

[n5ims]

The impedence mismatch using the preamp shouldn't be much of an issue, assuming it's on the receiver end of the duplexer. What you'll probably end up doing though is overloading the preamp from the very strong signal from the transmitter (the TV preamps are very poorly shielded and are very wide band so they'll pass nearly everything, including all of the noise and intermod.

A preamp for a repeater needs to be of very high quality and designed specifically for the band it's used on. Using a poor quality preamp is often gives you worse results than no preamp at all. While it may be worth trying, don't expect it to give you good results.

 

[centuryvrproductions]

While they are only like $30 at radioshack i might as well try it... i have no duplexer yet so i know that it will completey jam up if i tried it... anyway i am going to hook the preamp up to the reciever and to echolink so i can drive arround with my android echolink app and listen to the siginal as i key up the reciever. this will show me if the preamp improves the siginal. I am also using only a homemade dipole with 50ohm cable....

Ok if i have the 75ohm reciever amp inline from the reciever to the duplexer using 50ohm cable, would anyone know how much loss in db there would be with the mismach?

I bet its only like 1.5db or somthing, guessing

 

[SCPD]

Wirelessly posted (Mozilla/4.0 (compatible; MSIE 4.01; Windows CE; PPC; 240x320))

More like .15 dB. But I agree with the other poster that a TV preamp will probably make your signals worse rather than better.

 

[gewecke]

I Have a Repeater i setup on 440, I am still learning about antennas and stuff. I was looking to use a 10db TV antenna Preamp. It is 75ohms. I have a Motorola Maxtrac 50, and i was wondering if i could use the tv amp with 50ohm coax to the reciever (Maxtrac). Or will the impedence mismatch render the whole preamp setup pointless?

Do you really need the preamp?


73,
n9zas

 

[kb2vxa]

Unless you have problems with users who complain of being able to hear the repeater but are unable to access it you don't need a receive preamp. Let's hypothetically say that is the case so you need one but any broad band unit is a poor choice for reasons stated. There are preamps made specifically for weak signal work both in shack and mast mounted types, being for a single band they work extremely well. Bottom line here is IF you really need one don't skimp, this is definately a case of you get what you pay for.