DEMI 432 -28 Transverter with FM mod (Preliminary Report)

Roy Laufer

Here's my first moment's impression of the DEMI 432-28HP Transverter with a mod for the FM Repeater end of the 70 cm band:

The Amplifier is listed as 50 watts nominal. The instructions suggested driving it at a level when all but the last power LED was lit. My Power meter is a Comet CMX2300 (someday when Larry Phipps finally gets finished tweaking his LP-500 design, I will be using that for such measurements).

At first I set my transverter to 450.000MHz and tweaked the XVTR's Max Power to -5.3 dBm, where all but one power LED was lit). Here is the frequency vs. power approximately:

440MHz            <1 Watts
441MHZ             3Watts
442MHz             4 Watts
443MHz             6 Watts
444MHz             9 Watts
445MHz            11 Watts
446MHz            15 Watts
447MHz            21 Watts
448MHz            25 Watts
449MHz            28 Watts
450MHz            32 Watts

If you change the Max Power setting to its highest one (15.0dBm) you can squeeze out 10 watts at 440MHz - the higher frequencies are also driven to higher output power, but eventually you will overdrive the amplifier into distortion and see >40 watts output.

The pre-amplifier gain is much better in this model than DEMI's first attempt and I did not feel the need to slap on any further pre-amp gain thru the Flex6700.

The best game plan, as I now see it, is to place the Max gain much higher than -5.3dBm and "ride the gain with the RF Power control of the 6700 (this should be easier when/if I get a LP-500.

It seems like the folks at DEMI designed the filters to peak just at the 450MHz point - I would have chosen something more like 445MHz since Repeater transmit frequencies are equally scattered over the 440-450MHz range....

Well, it just came in a little bit longer than an hour ago, but I'm a "happy camper". Is it "perfect? What is? But you'll do, little 432-28HP, you'll do...

Any questions? (Keep in mind that my test equipment is presently close to non-existent.)

Ain't technology fun!!!

73,
Roy, AC2GS

Mark Gottlieb

Roy,

I ordered mine as well and am in line for  a great Valentine's Day present of a transverter from DEMI.  Anyway, can one attach an amplifier to this puppy to boost output without hurting anything?

73

Mark
WA2DIY

Mike

Roy: The unit has separate Transmit and Receive filters, I would expect DEMI tuned the TX for 445 to 450, and the RX from 440 to 445 most repeaters are looking for... Looks as if the transmit band could be centered better -- 73,Mike, WV2ZOW

Roy Laufer

I see no reason why not. If you are more interested in FM, rather than SSB linearity isn't as important a factor. Relatively decent amplifiers are available from Mirage (MFJ) and RM Italy. If you want the really nice stuff it will cost ya - i.e. Beko (like most things)

I plan to "play" with the transverter before deciding if an amp is necessary - I may get by with just riding the gain a bit...

Roy Laufer

I dunno - it all seems to be all over the place. I have Repeaters that would require me to transmit on 440.275MHz to 448.400MHz and receive from 440.050 to 449.625MHz!

Roy Laufer

Oops!

Please disregard all of the above numbers!

I thought that I would streamline the whole installation of the 432-28HP installation. I got all the connectors in place and labeled correctly. DEMI had advised me on the proper IF frequency months ago, so the XVTR was set accordingly (so much for pre-planning).

The drive power fall off was really bothering me, so I checked the enclosed documentation (RTFM) and noticed that the previously quoted offset was not the same as the offset this unit appeared to be designed for (there was a 5MHz difference!).

When I plugged in the proper values everything fell into its proper places. With the Maximum drive set to -6.0dBm and RF Power set to 100% it shows almost constant 32 Watts of output from 440MHz to 450MHz!

It's working like a charm! No complaints. The filter appears to be right in the correct place. I, was not in the correct place the first time.

Only goes to show doing something fast isn't nearly as good as doing it well.

Sorry for any confusion.

For those that want something more than 30+ watts (it is rated at 50 watts nominal), there is always the amplifier solution.

I will let you know how it is working for me.

Roy AC2GS

Mark Gottlieb

Do these numbers represent the low powered option for the DEMI 70cm transverter?

Roy Laufer

These numbers are based on my high power 432-28HP. Although their website claims 60 watts output power, the literature that came with my mod-ed transverter claimed 50 watts nominal output. DEMI's routine 70cm transverter is spec-ed at 25 watts nominal output.

Keep in mind these numbers aren't from a Bird Wattmeter, but a Comet SWR-Wattmeter, so the numbers might be a bit off.

The trouble with nominal power ratings is that they often disappoint (like hard drive capacity figures).

Mark Gottlieb

The reason I ask is because I was thinking of downgrading to the LP version and just getting a good 70cm amp (TE System or RM Italy) to do the job.  If there is a 50% drop off the promoted output, I just as well go with the LP version.

Roy Laufer

That certainly is a reasonable choice. The low power version is rated at 25 Watts nominal output, which means it might end up being closer to 18-20 Watts output. I thought that I might get by with 30+ watts, but if I find it's not, I can always get an amplifier.

If you plan on an amplifier from the get-go, you'll save a few bucks going with the lower powered transverter version.

EA4GLI

Do you have a chance or the equipment to record a video of the transverter in operation? It would be great if you can post it on youtube.

Roy Laufer

There's not that much to see. A DEMI box that lights up when I transmit...

Then there is the panadapter with the upper portion of the 70cm band being displayed...

The transfer switch, itself, is about the size of three matchboxes stacked on each other and it is lost somewhere in the back. The only indication that something strange might be going on is the small inline rocker switch to energize the transfer switch's relay.

Is there any specific aspect that you are curious to see?

EA4GLI

Ideally a wide angle of the equipment and then zoom in on SmartSDR on the screen while you work a local UHF repeater.