Better to run an all-mode 2m for transverter?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K3RW, Oct 13, 2017.

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  1. K3RW

    K3RW Ham Member QRZ Page

    I'm looking at some transverters for the microwave bands, and I've done some SSB contesting on 2m and 432. Current rigs are a ICOM-7100 and a pair of 706MKIIGs, as well as a 7200 (160-6, no FM), and a 7600 (HF + 6m all mode, but no VHF/UHF).

    Obviously I will downsize at some point very soon :)

    My rig drifted a fair amount on 432 during the latest contest, on both the 7100 and the 706MKIIGs. At first I figured the other end was using a tube transverter. Nope. We chatted about it this week and he had a similar issue with the 706MKIIG until he installed a CR-282 TXCO. Then the drift he saw went to about nothing.

    By drift, I mean, I was 20 below on the net frequency last week when I thought I had it dead-on 2m SSB (according to NCS), and kept upping the RIT until I realized it was also my TX end. By the end of a 1 hr net I had swung almost 75 total just to keep up with him. During a long contest, I'd be way off. A fan mod will help no doubt.

    So, frequency stability is always an issue with transverters. And I pondered, am I better of to get a dedicated 70cm and/or 2m all-mode rig so it doesn't have the huge multiplication problem? Would these potentially have better rx sensitivity? Looking at noise specs, I really don't know how to tell. But then, I see others that use a 10mhz clock, and I wonder if this mitigates things.

    Also, I noticed that both ICOM and Radio Shack made the CR-282, and perhaps some others did too. Any reason I should get one or the other? The RS ones are a heck of a lot cheaper.

    Seems I can go the TXCO route, or go with a dedicated 2m 432 rig. A few transverters I've seen (the Ukraine 220 one) use 28mhz IF.
     
  2. W0AAT

    W0AAT XML Subscriber QRZ Page

    TXCO will fix it, and I do not worry about dial frequency, as long as I am on for receive for the net that is all that counts. I have yet to see a 2m all mode that is dead on freq from the factory! I typically run about 144.199.95 on my radio for someone on 144.200 for example... but his radio may be high... it happens. As long as drift is controlled that is what counts!
     
    K3RW likes this.
  3. K3RW

    K3RW Ham Member QRZ Page

    Thanks for the pointers. I got sticker shock looking at the ICOM 9100, and much older VHF all-mode rigs are commanding a fortune on the resale market. Yikes!

    The multiplication may not a problem unless I try to hit 1296 repeaters--and I'm really not planning on that. Anything else I can spin the dial and it will likely just be skeds.

    Would I be better off running the 706MKIIG with the TXCO, or the 7100 as-is? I haven't seen a 7100 TXCO mod, and perhaps it is already much more stable. I haven't been able to do a A/B test between them to notice if the 7100 drifts as much, or at all.

    Sort of related, I did a SOTA activation last weekend. We tried 1.2 ghz FM (not on a repeater) and I was surprised it worked as well as it did. We did have to spin the dial a bit, but the guy with the transverter said that FM is a bit more forgiving as a mode. Hadn't heard that before.
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I'd say, "No, it's not." It's much more an issue when you get up to the microwave bands, and it's common to use Rubidium standard locked oscillators for transverters at 5.7, 10.3 and 24 GHz. But at 432 MHz? I've never had any kind of "drift" problem using good crystals in the commonly available transverters; generally they're factory trimmed to be within 100 Hz or so, and even a few years later they're still about there. I've used transverters on 70cm since the late 1960s. Even my Yaesu FT-736R (multi-band, multi-mode VHF-UHF transceiver) actually uses "transverters" inside, although they're not external. I bought that new in 1987 and now 30 years later it's within 100 Hz or so on 70cm and doesn't drift.
    An optimized transverter will almost always outperform any kind of HF+VHF+UHF rig's receiver, with the possible exception of the Icom IC-9100, which I've used quite a lot and it's really good (it's also $2,200). And of course the old FT-736R is also excellent but is only a VHF-UHF rig with zero "outside the amateur band" coverage -- that way it can be well optimized for both RF (front end) selectivity and noise figure within the band ranges.
    Radio Shack didn't make TCXOs. However a lot of battery types begin with "CR." Are you sure the RS product you've looked at isn't a battery?
     
  5. WB3BEL

    WB3BEL Ham Member QRZ Page

    The IC-706 is a pretty drifty radio on the V/UHF bands. A TCXO will make it better but so will keeping the radio indoors in a somewhat temperature controlled environment and keeping a fan on it so it does not heat up too much when transmitting.

    These radios are small and get quite warm when just receiving so they drift quite a bit from a cold start.

    Many operators have deemed them unsuitable for high stability requirements like V/UHF digital EME.

    But the reality is that you can keep it away from other equipment and put an external muffin fan on it and after 15 minutes or so of warm up it will be as good as most for medium term stability over minutes needed to complete EME. A TCXO plus these steps would be better.

    But before I spent a lot of money on TCXO, I would seriously consider this cheaper alternative:

    https://shop.kuhne-electronic.de/kuhne/en/shop/accessoires/crystal-heater/

    This approach is kind of old tech but works suprisingly well.

    I think there are not too many folks using rubidium sources for portable microwave work these days. There are much better alternatives. Rubidium sources wear out over time. They can be big and use some power. Instead, you can buy GPS stabilized 10 MHz source from Ebay for less money and it will have better stability most of the time if you can receive GPS signals. $100 give or take. There are lots of sources that came from decommissioned 2G and 3G mobile phone base stations. These are what I would recommend for a microwave converter timebase if you want to know for sure what frequency you are on. Where the other guy is a different matter!

    One of these GPSDO 10 MHz sources can be tripled to 30 MHz and externally injected as a reference to the IC-706 if you don't mind making modifications.

    The IC-706 does not have good dynamic range on 144 MHz so is a pretty crummy choice for contest use on mountaintop. But for normal weak signal digital work or EME, once you fix the frequency stability issue it works just fine.

    Most of the microwave amateur bands are not so jam packed that high dynamic range is an issue except when you are near some big mountaintop operations on contest periods. So as long as you don't set up near one of these stations and point your antenna their way something like the IC-706 should be fine for microwave IF radio. An SDR might be better for IF receiver as you can see stations pop up when they are not on the expected frequency and so far off that you would have to tune to even find them otherwise.

    Good Luck with your experiments,
    Harry
     
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  6. K3RW

    K3RW Ham Member QRZ Page

    Interesting observations. Thanks for all that.

    I'm struggling to understand the dynamic range concept. Is it partly because it is a HF rig that just happens to have V/UHF capability? Would a different rig work better--say the 7100?
     
  7. K3RW

    K3RW Ham Member QRZ Page

    I took a second look at this supposed Radio Shack TCXO... here it is:

    http://www.ebay.com/itm/CR-282-Comp...998712?hash=item1c74489778:g:m0QAAOSwX9FZHrr7

    Here's what the description says:

    RS-282High Stability Crystal Unit

    COMPATIBLE ICOM CR-282

    When the optional RS-282High Stability Crystal Unit is installed, you get a very high frequency stability of ±0.5 ppm (-30C to +60C).

    Compatible with Icom radios:
    IC-706MKIIG IC-765 IC-736 IC-775II IC-746 IC-R72 IC-R75
    --

    It looks a lot like the ICOM one. But I notice they are saying this is a RS-282, "compatible CR-282". I can't tell if this is just a plug-n-play type swap, or if it is really a 'take it to a shop' type mod. One video I watched on Youtube made it look really complicated, but never actually showed if the crystal just pulls out or not.

    Yep, the 9100 is a neat rig. But wow, makes the 736R look super cheap. Those are superb, but definitely out of my budget. Someone needs to make another all mode dual band rig. Or just make a CW version--I could get by with that too. Looks like I'm stuck in 7100 or 706 land for the time being, but at least I have some options.

    Still not sure if the 7100 is a better way to go, ultimately.
     
    Last edited: Oct 14, 2017 at 3:58 AM
  8. W0AAT

    W0AAT XML Subscriber QRZ Page

    I know someone who used a 706MKIIg for EME for years... it can be done. New digital modes it might not be stable enough for but cw? Yup!

    I use Yaesu FT-991's and am happy with them, no drift issues I have seen...
     
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  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    No it's not directly because it is an HF rig with V/UHF, although many of these all band radios by other vendors also have poor dynamic range.

    It is because even the IC-706 HF receiver has quite poor dynamic range. Many operators use them with mobile or other portable antennas in everyday use just fine.
    The VHF/UHF segment has poor dynamic range partly because it is built on top of the crummy HF sections and partly because it does not have good filters or good dynamic range V/UHF components either.
    It was meant to be a compromise and it is. But you can still work lots of interesting stations with them if you know how to get the most out of it. I have one and I like it fine and use it quite a bit on V/UHF.
    I have made hundreds of digital EME contacts with the 706mk2g, hundreds of digital meteor scatter contacts and thousands of 6meter Eskip QSOs.

    You need good dynamic range when there are lots of strong signals in the presence of weak ones that you want to hear.
    Operators with good, big antenna systems up high often operating in contest or DXpedition use cases see lots of strong signals when they are trying to receive very weak ones.
    This is the kind of use case that requires good dynamic range.
    If you have a poor antenna and just work casually what you hear then dynamic range is maybe not your highest concern.

    Here on the east coast if you take a radio like the IC706 to a mountaintop for V/UHF rover work and you are close to one of the big gun stations during a major V/UHF contest you will quickly find that it is very easily overloaded when you point your antenna toward that mega station and you can not hear weak signals in their direction if they are close to the same frequency. But if you are a rover you can choose where to stop and how long to stay there so this kind of limitation is not the end of the world. It's really only a major pain on 50 MHz and 144 MHz.

    On the microwave bands almost all of the signals are pretty weak and antennas are usually much more directional so microwave overloading is something that does not occur as much and you have more ability to adjust things to make it less of a problem. So these kind of portable mobile rigs work pretty well for microwave IF with transverter. But as I mentioned before if I was starting from scratch on the microwave bands I would strongly consider an SDR based IF solution which allows you to see instantly much wider segments of spectrum without turning knobs. This is also very useful for VHF/UHF work where short openings may occur off of the calling frequencies.
     
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  10. K3RW

    K3RW Ham Member QRZ Page

    Thanks so much. I like the twin passband filter of my 7200 and it has 3 filters. Works well until the beam guys start running full limit in the RTTY contests. The 7100 has something similar but its tougher to use.
     

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