Using a TV Preamp for a repeater?

[centuryvrproductions]

I have a Ham Repeater on the 440 Band and i need a preamplifier on the reciever. I dont want to spend alot of money so i was hoping to use an antenna Preamplifier for an antenna tv. I saw that they were on the same band and around 16db gain...

Repeater
RX-449.500
TX-444.5
KB1VSP Repeater

 

[N0BDW]

I have a Ham Repeater on the 440 Band and i need a preamplifier on the reciever. I dont want to spend alot of money so i was hoping to use an antenna Preamplifier for an antenna tv. I saw that they were on the same band and around 16db gain...

Repeater
RX-449.500
TX-444.5
KB1VSP Repeater

The impedance likely does not match (50ohm for ham stuff, 75 for TV). I would suggest verifying that before attempting.

 

[centuryvrproductions]

Kb1vsp

I am using HT's and the antenna is 50ohms... is there any way it will work? i have HT's for the repeater RX/TX...
and i do believe that the preamp is 75ohms

 

[N0BDW]

It may "work," but the impedance mismatch may negate any benefit you'd see by using it.
As always, YMMV.

 

[zz0468]

There's a few things to consider before you do this:

First, the noise figure of the preamp must be lower than the noise figure of the receiver. If it's not, it will harm, not help, weak signal reception. Strong signals will get stronger, and weak signals will just go away.

The next thing to consider is the third order intercept (IP3) specifications of the preamp. That measures the ability of the preamplifier to exist in a high signal environment (the repeater transmitter) without going into non-linear operation. Once that happens, the preamp becomes an intermod generator. Oh, yes... and the noise figure goes to hell.

The next thing to consider after that is, how much gain can you put in front of the receiver before IT goes non-linear and becomes an intermod generator.

The last thing to consider is, when you improve receiver sensitivity, assuming the preamp's specs are sufficient for what you're trying to do, it will require increased isolation between the transmitter and receiver to avoid desense.

 

[rfradioconsult]

Impedence is not a problem very minimal (read unnoticable) between 50/72 impedence, a TV type broadband amp will eat you alive because of it's poor IM performance. There are commercial RX amps available with tuned input/tuned output that may or maynot help for your specific situation. Have you checked your receiver to determine what the RX sensitivity really is?

 

[davidgcet]

if you are using HT's i am willing to bet the issue is either the RX, a bad conneciton, or a mistuned duplexer. do you have access to a service monitor that will do duplex gen or at least a regular service monitor and an iso-tee?

 

[AK4GA]

In before the interference letter.

 

[AK4GA]

if you are using HT's i am willing to bet the issue is either the RX, a bad conneciton, or a mistuned duplexer. do you have access to a service monitor that will do duplex gen or at least a regular service monitor and an iso-tee?

YouTube - ‪Echostation Repeater KB1VSP‬‏

I don't see a duplexer.

 

[AK4GA]

You know, I'm sorry if I am coming across as a dick but this in its current course is likely to cause a situation that is a current sore spot with me.

Having said this, I know OP is a new ham and may have some apprehension in asking for help. I will offer my hand to the OP if he will accept it and maybe we can get him on the right track.

What do you say Victor?

 

[k4sgt]

I recently added a Down East Microwave Inc. 70cM RX preamp to my repeater. In kit form it is only $45 plus shipping. If you feel that a surface mount kit might be too much of a challenge then you can also purchase it pre-built for $75. Performance has been absolutley awesome for me. The kit you want is the L432LNACK or the built version is the L432LNA.

Catalog Frame page

 

[davidgcet]

so is it a store and forward in 1 HT or is the actual repeater not shown in the clip? if it is S/F in that radio then you would have to wire in a preamp on the RX side of the antenna switching network or else the first time it TX'ed it would fry it. if it is a full duplex pair of radios that was not shown, how far apart are the antennas as the TX could be desensing the RX.

 

[prcguy]

There is a lot to consider when adding even the best available high level preamp to a repeater. Hopefully the early posts in this thread have ruled out using a TV type preamp for obvious noise figure and overload reasons.

zz0648 touched on isolation issues and as a general starting point I would be looking for at least 75dB isolation between transmit and receive frequencies from the duplexer. A basic notch only duplexer will usually give 75dB isolation but if you add a preamp with 20dB of gain you have increased the noise floor contribution from the transmitter by about the same gain as the preamp and have impacted the isolation by about the same amount.

When adding a preamp its usually good practice to add some front end filtering to provide at least as much isolation between transmit and receive frequencies as the preamp has gain in order to preserve or improve the transmitter to receiver isolation.

The drawback is the insertion loss of the filtering will add directly to the noise figure of the system but if the repeater is located at a site with other transmitters the noise floor in the area is probably much higher than the added noise figure of additional front end filtering before the preamp.

I just ran into this problem again a few weeks ago with a VHF repeater on a very busy mountain top in So. Cal. The repeater receiver I was dealing with will easily meet spec at about .22uv or -120dBm but the constant noise floor at the repeater site on our frequency is about -100dBm or 20dB worse than the repeater is capable of hearing. In effect the repeater receiver has a 2.0uv receiver sensitivity which is crap.

I did install one of Angle Linears best super high level VHF preamps with a 3 cavity preselector in front of the preamp and the end result was slightly better coverage with handhelds on the system. I believe the improvement is from the handheld signals (and noise) being raised to a level where the receiver is into full limiting and better quieting despite the poor signal to noise ratio.

Had I done this with a TV type preamp it would probably degraded the system by more than what the Angle Linear improved it.
prcguy

 

[rfradioconsult]

The OP needs to provide some additional information, the overall consensus is a broadband TV pre amps will not work, aside from that we know nothing about his configuration or some idea about his ambient RF environment. He did mention he is using portables at the TX RX stages, which already can be problematic with poor recieve performance.

 

[n5ims]

The OP needs to provide some additional information, the overall consensus is a broadband TV pre amps will not work, aside from that we know nothing about his configuration or some idea about his ambient RF environment. He did mention he is using portables at the TX RX stages, which already can be problematic with poor recieve performance.

Basically (according to this thread anyway http://forums.radioreference.com/am...cussion/212873-building-ham-repeater-hts.html) the OP is trying to make a repeater using two HTs. They want to place a preamp on the receiver to increase reception and put a 200 watt amp on the transmitter, also to increase range.

 

[rfradioconsult]

If he used GE radios to build the repeater, would that make it a KLUGE!

 

[AK4GA]

If he used GE radios to build the repeater, would that make it a KLUGE!

Whats wrong with a GE repeater?

 

[LtDoc]

What's wrong with a GE repeater? The only thing wrong with them is whatever is actually wrong with them. If nothing's wrong with it, then nothing is wrong with it.
The typical sensitivity of a hand held radio isn't exactly lacking. What's lacking is a decent antenna with some height to it. No, I don't mean hold the thing over your head. Unless your arm happens to be 50, 100, or 300 feet long, or whatever it takes to get it above the average surrounding terrain. sensitivity isn't the problem, but selectivity might be.
If we are talking about a VHF/UHF repeater, then 200 watts is more silly than useful. Unless you plan to interfere with every other repeater on the same frequency within a days driving range, sort of. Depending on propagation, a local repeater get's 'interference' from a repeater over 200 miles away. (I think the power output is in the 20 watt range.) Of course, if that antenna is at arms reach height, use that 200 watts, you'll need it, maybe.
The biggest problem with using hand-held radios as a repeater would be heat dissipation (even those GE's). You have to figure on 100% duty cycle, so keep the power level at a minimum. Come to think of it, that would go for an amplifier also. Next biggy would be supplying primary power. I figure that alone would destroy any 'portability' factor of a hand-held repeater, batteries just aren't too 'portable'. If this repeater is for a "have to" type situation, the problems with primary power just doubled, or maybe quadrupled?
There's always something, right??
- 'Doc

 

[rfradioconsult]

Not a thing wrong with the GE family of equipment I have experiance with GE dating back to the pre-prog days, including the TPL series up to the MASTR III products. I was trying to determine if the OP was using a duplexer or separate antennas.

 

[davidgcet]

i see someone tried to improve the GE repeater with a MSR/PURC power supply!

GE repeaters are actually really robust. other than direct lightning strikes, i have had several that ran for a decade or more with no issues.