Narrowband Splinter Channel Repeater from two TK-730G Radios

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aaknitt
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Joined: Wed Oct 05, 2005 4:30 pm

Narrowband Splinter Channel Repeater from two TK-730G Radios

Post by aaknitt » Wed Aug 29, 2007 9:36 am

I'm wondering if it would be possible to build a narrowband-compliant repeater operating on a 7.5 kHz spaced channel with two TK-730G radios by purposely misaligning them both to be 2.5 kHz off of the programmed frequency. Since it would be a repeater, only one channel is needed and therefore being offset on other channels wouldn't be an issue. A few questions:

1) Is it possible to purposely misalign the radio to be 2.5 kHz up or down from the programmed frequency?
2) If so, where can I find the procedure and/or service manual? (I have access to the needed test equipment)
3) Would this be legal?
4) Is there way to turn down the power on a 110W unit (to 50W or so) to increase duty cycle for repeater operation?

Thanks in advance,

Andy

k2hz
Posts: 150
Joined: Sun Jan 30, 2005 1:02 pm
Location: Rochester, NY

Re: Narrowband Splinter Channel Repeater from two TK-730G Radios

Post by k2hz » Wed Aug 29, 2007 10:04 am

Your idea of warping the reference oscillator frequency to shift the channel frequency is theoretically possible in the situation you describe. The limiting factor is probably how far the frequency can be shifted.

Motorola sold my former employer a SCADA system for an odd 1.25kHz split and they did it by that oscillator warping technique. The main problem was that, after the first shippment, the radios were delivered without the offset and anytime a radio went back for depot service, they "fixed" the offset so the frequency was wrong.

I don't have a manual handy for the 730G so I am not sure how the reference oscillator is adjusted. I think 2.5kHz is a bit much but you don't know until you try.

Since the radio is type accepted for narrow band, the reference oscillator shift should be legal as long it is winds up stable on the assigned frequency. That was Motorola's position for the system I mentioned above.

There is more of a legal issue with the power reduction. I don't know if the 730G is type accepted for that much power reduction. In any event, you would need to do a spectrum analyzer check since many radios go spurious when power is reduced more than about 25%.

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ohgary
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Re: Narrowband Splinter Channel Repeater from two TK-730G Radios

Post by ohgary » Wed Aug 29, 2007 1:20 pm

Would such a radio be type accepted to use on any service if your misaligning the radio. I doubt it would be legal in services requiring type acceptance, but would be legal on the ham bands.

k2hz
Posts: 150
Joined: Sun Jan 30, 2005 1:02 pm
Location: Rochester, NY

Re: Narrowband Splinter Channel Repeater from two TK-730G Radios

Post by k2hz » Wed Aug 29, 2007 2:44 pm

I would agree is is not generally a good practice and it must be kept in mind it works only for a single frequency application since the shift will mess up other frequencies.

The opinion I got from the Motorola legal people when I questioned their solution in the case I mentioned above is that is legitimate to do a service adjusment to shift the frequency to an offset necessary to be on your licensed frequency (like adjusting the xtal trimmer in the old days). But, it probably would not be legal if you made any component changes or other modifications to force the offset you needed since this could affect frequency stability.

All things considered, I think the best solution is to get a new radio that is designed for the necessary frequency splits.

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