70cm Preamp Help

Discussion in 'Satellite and Space Communications' started by N7AY, Jan 14, 2022.

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  1. N7AY

    N7AY Premium Subscriber QRZ Page

    The ic9700 is full duplex. What you describe is how I was anticipating the SSB-Electronics device to work with the ic9700. Does it work that way? As for weather proofing, I was thinking of coating the device in that liquid rubber used for coating the handles of tools. But I'm concerned about cooling (if needed) and condensation.
  2. N0ZGO

    N0ZGO Ham Member QRZ Page

    Every dB of loss in that feedline at 70 cm adds directly to your system noise figure (NF). The IC-9700, as measured by AB4OJ, with the internal preamp on has a NF of 4dB.
    A mast-head LNA with a ~1dB or less NF and ~20dB of gain will set the system noise figure and make the coax you use largely irrelevant for receive.

    Here's one of many options:
  3. KS1G

    KS1G Ham Member QRZ Page

    My preference is mast-mounted preamps that come in weather-proof enclosures and include RF switching. More $$, but I am much less likely to blow one by an accidental transmission. I power them through extra conductors in the rotator control cables, and have a switch box in the shack to control the power. The RF protection saves me when I forget after switching between VU and UV satellites! I don't really need one on 2M (the one I have is a relic from AO13 days!), but they are very useful for RS44 and other VU satellites. ~40-50ft of LMR400 to between radio & preamp, then a rotor loop of LMR4ooUF.

    SSB is good, but no 1st-hand experience, I purchased from ARR when they were still making them. (They still repair them if you happen to blow one up!) I just listened to a 10-15deg RS44 pass out over the Atlantic with and without the ARR masthead preamp - HUGE difference, the 9700 internal cannot make up for it and doesn't help the noise figure.

    If you get an non-switched preamp, you will need to put RF relays around it to protect it, and have a fail-safe sequencing mechanism in the shack that delays radio PTT until the protective relays have switched. The serious VHF/UHF weak signal ops have this all figured out.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's "LMR400," and sure it's easy to upgrade to LMR600 which has about half the loss and isn't terribly expensive (although it requires special connectors).

    If you're using diplexers to use one run of coax on both bands to separate antennas, they will each have about 0.2 dB loss on 70cm, so there goes 0.4 dB. Doesn't seem like very much, but then the S/N improvement available with a preamp is only whatever loss the connection system has from antenna to radio, unless the radio's NF is pretty bad (the 9700 isn't bad at all).

    Don't think if you have a "20 dB gain" preamp that signals will be 20 dB stronger. On "receive" the only thing that counts is S+N/N, so "X gain" does not result in "X" improvement in that. The preamp only needs enough gain to overcome system loss (diplexer, feedline, connectors) and the NF of the receiver used. About a 1 dB NF is all that's required on 70cm if looking at a pretty dark sky; 2 dB may well be good enough in most cases as noise generating stuff (sky noise + more local effects like foliage) may exceed that.

    I'd always opt for minimizing all system losses first. With a short transmission line and a good receiver, a masthead preamp may not help. Of course, your particular 9700 "might" have a problem I couldn't know about.
    KG4RRH likes this.
  5. N7AY

    N7AY Premium Subscriber QRZ Page

    Thanks all for the input. First I'm going to clean up the feed-line - get it to just one run of LMR400 per yagi and no diplexers.
    I'm also going to clean up the feed-line run and antenna mounting method as per instruction from M2.
    Then, if it still needs help I'll look into a preamp.
    VE4MM likes this.
  6. KS1G

    KS1G Ham Member QRZ Page

    How long is your run end-to-end, and are your antennas mounted in a permanent location? My setup is "technically" non-permanent (thank you so very much HOA /sarc). I would seriously consider buried hardline if I could put up even a short tower. Alternatively, the cost delta for LMR600 (or equivalents) may be worthwhile if you are going to buy more coax anyway (at least for your UHF run). I borrowed 2 100ft runs of LMR600 for an ARISS school contact in 2019; a Bird wattmeter could barely detect any signal loss!
  7. W9GB

    W9GB Ham Member QRZ Page

    Times Microwave LMR600 is used extensively at mobile/cellular sites.
    We are in the middle of the Mobile 5G upgrades (Verizon & AT&T just activated their next implementation phase). SO, Surplus Tools and Cabling will be available in the near future.
    Look for those spare coax spools rolling down the street..

    About 15 years ago (end of 4G/LTE upgrade),
    I picked up a surplus RFI Industries Pro Crimp Tool (RFA-4009) and
    ā€œNā€ connector Die for LMR600 coax.
    It was dirt cheap. Some crews tossed their crimp tools and connectors when contracts finished.
    Last edited: Jan 18, 2022
  8. N4UFO

    N4UFO Ham Member QRZ Page

    I'm going to remind you of my suggestion in a previous post, of using those diplexers as filters to keep 2m third harmonic out of your uhf downlink. (That and front end overload.) https://forums.qrz.com/index.php?threads/feed-line-and-preamp-help.793567/#post-6057543

    If you can find a switchable preamp for UHF (new or prefereably used SP432VDG), I think you will see a WORLD of difference. First time I switched mine on, the improvement was incredible. (granted, I had 50+ feet of less than optimal coax)

    By the way... I hope you are/were NOT using MFJ-4916s diplexers like you show in your drawing in that thread. Because if you are/were... you may have found your problem. That model is for HF/VHF and NOT VHF/UHF. o_O
    Last edited: Jan 19, 2022 at 10:19 AM
    KB0MNM likes this.
  9. KB0MNM

    KB0MNM Ham Member QRZ Page

    Just curious:
  10. N7AY

    N7AY Premium Subscriber QRZ Page

    End to end it's 72' with no reasonable way to make it shorter. I'd actually like to make it longer as I want the yagis on a taller mast but it will do for now. About 50' of the feed line run is in a 1.5" buried conduit. Location is permanent. With the antenna gain and the ic9700 I seem to have plenty of TX power. 70cm receive that's my weak spot. I already have enough LMR-400 to make the run and enough room I think to fit it in the conduit. Seems like all I need is a switching preamp and I'll arrive at the next performance plateau.

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