Batboard

I have always thought, if it looks too good to be true....
On that basis, I have noted on ebay that there is a Power Seller in Hong Kong flogging a 6 cavity 50 watt VHF high band duplexer "new" for $100.00 + delivery.
Has anyone here used on of these ? If so, any thoughts appreciated. I only need one to run at 5 watts and they sure cost a lot more than that.

Thanks in advance
Mike

What's the minimum split? For that price, I'd expect around 6 MHz or better. If the guy claims 0.2 MHz, or some other ridiculous claim, I'd steer clear. For that price, if they do give that kind of split, I'll practically guarantee they won't hold their tuning more than six months, less in humid conditions.

Here are the specs he is quoting on the unit concerned.


50W RF power Handling

6 cavity Design

Minimum Tx and Rx frequency difference: VHF 3.5Mhz

Maximum Tx and Rx frequency difference: VHF 15Mhz

Power Isolation >75dB

Power loss <1dB

Easy to build your repeater

Support Frequency: VHF 135-175Mhz

Free Frequency Tuning - send me the high /
low frequency

N connector

Fit for Motorola Repeater Enclosurer


Any further thoughts appreciated.
Regards
Mike

2Kg, (15.5 x 22 x 3)cm
Delivery Time 2 week

Not the best specs in the world. Price still seems a bit low even for them. My personal advice is leave them alone. Usually, you get what you pay for with this kind of thing.

mrtor wrote: 50W RF power Handling

So pretty much useless for a repeater of any real power output.

mrtor wrote: 6 cavity Design

Minimum Tx and Rx frequency difference: VHF 3.5Mhz

So useless for a repeater in the 2M amateur band where the split is 0.6MHz

mrtor wrote: Maximum Tx and Rx frequency difference: VHF 15Mhz

I'm not sure why there would be any limit on the MAXIMUM split....

mrtor wrote: Power Isolation >75dB

So if you are running 50W (47dBm) and you get 75dB isolation, you will be getting -28dBm into your receiver. Can you say "desense"? I knew you could. (and if they were getting any more than 75 dB isolation they'd spec it so.)

mrtor wrote: Power loss <1dB

This is the only spec I've seen that was worth a darn, and give the relatively low Q of these cans (given the minimum split) I'd hope they weren't too lossy.

There's no magic to a cavity filter, so I could believe that China could make good ones cheaply - the biggest cost would be in the materials. But these cavities aren't good ones.

Wowbagger wrote:

mrtor wrote: 50W RF power Handling

So pretty much useless for a repeater of any real power output.

mrtor wrote: 6 cavity Design

Minimum Tx and Rx frequency difference: VHF 3.5Mhz

So useless for a repeater in the 2M amateur band where the split is 0.6MHz

mrtor wrote: Maximum Tx and Rx frequency difference: VHF 15Mhz

I'm not sure why there would be any limit on the MAXIMUM split....

mrtor wrote: Power Isolation >75dB

So if you are running 50W (47dBm) and you get 75dB isolation, you will be getting -28dBm into your receiver. Can you say "desense"? I knew you could. (and if they were getting any more than 75 dB isolation they'd spec it so.)

mrtor wrote: Power loss <1dB

This is the only spec I've seen that was worth a darn, and give the relatively low Q of these cans (given the minimum split) I'd hope they weren't too lossy.

There's no magic to a cavity filter, so I could believe that China could make good ones cheaply - the biggest cost would be in the materials. But these cavities aren't good ones.

Good Morning Wowbagger thanks for that assesment. As I stated at the top, my philosophy in life has been "If It Looks To Good To Be True, It Probably Is". I've beem proved wrong in the past and I know that China can produce some remarkable really good stuff really cheap. This appears to Not Be one of those really good things. I appreciate your thoughts, and will save my dollars to buy a "Proper" duplexer.

Regards from Oz
Mike

Wobbager wrote
:So if you are running 50W (47dBm) and you get 75dB isolation, you will be getting -28dBm into your receiver. Can you say "desense"? I knew you could. (and if they were getting any more than 75 dB isolation they'd spec it so.)

But you forget, It would only be -28dBm into your receiver if all of the 50 watts of your transmitter was on the same freq as the receiver and then attenuated thru the duplexer by the duplexers 75 dB spec.

At some frequency seperation away from the carrier frequency the broadband noise that falls on the receive frequency would be much lower than +47 dBm, and typically around -23 dBm or lower.

If you take a typical PA where the spurious level even a couple of MHz away from the TX frequency was lets say 70 dBc (70 db below the carrier level) on the receiver frequency, which is not unusual, then the tx noise at the rx frequency would only be be at -23dBm not +47 dBm.

Adding in the duplexers attenuation of 75 dB then this should knock this -23 dBm signal on the RX frequency down further to -23dBm + -75 dB or to a level of -98 dBm which is still really crappy receiver sensitivity if you want to hear anything.

Either way these sound like cheap mobile duplexers, even if you were only running your TX at 5 watts, with the same assumed spurious level out of your TX of 70dBc, then this would put your tx noise on the receive channel at -33 dBm and with the added 75 dB duplexer attenuation you would still only be at -108 dBm useable sensitivity for the receiver which is still crap.


And yes I know that most motorola PAs will have much better spurious specs than 70 dBc at even +-500KHz away from the transmitter frequency but I was taking the FCC spec for spurious out of a transmitter which I think is still 70dBc (70 dB below carrier level).


M