Mast Mounted 2m Preamplifier Recommendations......

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K3GM, May 25, 2015.

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  1. K3GM

    K3GM Ham Member QRZ Page

    ..by the way. Unless you guys advise differently, the plan is to power the preamps in parallel. There will be sufficient voltage and current to feed both at the same time.

    Also... do you guys think I could hard key both simultaneously regardless of which antenna is active at that moment? The preamps have a capacitor of sufficient size that it will hold the T/R relay open until there's sufficient decay in voltage where it drops open to Receive. The idea is that you can run QSK CW without the relay following the keying. That a pretty nice feature, I think
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I see no reason not to power the preamps in parallel.

    If the relay hangs due to a capacitor charge, then what you're doing isn't really QSK.

    Earlier I posted about a 20 mS delay for sequencing and you responded with 'another missed contact.' I assume that was a joke, as you can't miss a contact with a 20 mS delay. Relays all have switching time and it's often 7-10 mS even for pretty fast relays. I buffer that to about 20 mS purposely to be sure contacts are fully settled, and I think almost everyone does this.

    An alternative would be to ditch the relays entirely and use PIN diodes, which at VHF work very well and can have even less loss than relays do; but good ones are pricey, especially if you run 'power.' My current 2m T-R switch in my KW amp uses Unitrode PINs with 2A of forward bias on TX and 10 mA of reverse bias on RX for the forward path devices, and that takes a switched dual-polarity power supply but it accomplishes less than 0.1 dB TX loss and about 55 dB RX isolation, so it's at least as good as a very good coaxial relay but no moving parts and more reliable.

    Unfortunately that was not a money-saving exercise. The damned switch cost nearly 250 bucks by the time it was built.:p
     
  3. K3GM

    K3GM Ham Member QRZ Page

    Thanks, Steve. I guess I should clarify. I can leave my K3 in QSK mode and not have to switch it back and forth when going to 6 and 2. Also, it was not I who made that comment. I had asked if setting up a delay in the K3 upon transmit would suffice, and you replied affirmatively. So:
    A: Sounds ok to power both amps in parallel
    B: The K3 will control both preamps in parallel as well, switching both units from Receive to Transmit regardless of which band I'm using.

    I have one unused wire in the cable that could be used to control each preamp individually if I had to. But it further complicates things back in the shack. There is a possible that I would move the 6m band off the K3 to an Apache Labs ANAN-100, and then I'd need that wire. But I don't know if I'm ready to use the ANAN yet in a contest situation... Maybe soon....

    Thanks!
     
    Last edited: Jun 21, 2015
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I was the one that said 'another missed contact.'.

    Sorry for the bad joke.


    Happy Fathers Day.

     
  5. KB0TT

    KB0TT Premium Subscriber QRZ Page

    The Method

    This has worked before and SHOULD work now .... The method Steve describes , more or
    less , the same pin diode arrangement and delay of ~ 20 ms that K4VX ( Hannibal , Missouri )
    used ~ 1976 ....

    Bottom Line : It worked ..:cool:


    JB
     
  6. K3GM

    K3GM Ham Member QRZ Page

    Thanks for the ideas. In my case, the T/R relay is contained within the preamps (I went with the 6m preamp too!), and perhaps I'm not actually using its proper name. The relay actually switches the preamp in and out of the circuit. So if I'm reading the schematic correctly, with my K3 in the full-break mode, I hear the receive signal bypassing the preamp. When I stop transmitting, the 47uF cap discharges sufficiently, the relay drops out, and the brings the preamp back into the circuit. I don't know yet if this unamplified>>>>amplified effect will be annoying or not, but we'll find out soon. I suppose if the preamps were older units, I'd be more willing to take the lids off and mess around with them.
     
    Last edited: Jun 24, 2015
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A simple electronic sequencer is 'adjustable' and can control many devices.

    I'd recommend one, it's what almost anybody using masthead preamps and serious about weak signal work does.

    You can set the timing of everything any way you wish using one. With mine, I set it so the PTT signal from the rig first keys the relay in the preamp, then 20 mS later keys the amplifier in the shack, and 20 mS later allows the rig itself to transmit; and the sequence is reversed when going back to receiving. Keeps all the smoke inside stuff, and if I want to set it for 10 mS or 30 mS instead, easy to do in about ten seconds, in the shack. Mine has five output lines with separate adjustments so it can control stuff on five different bands. But it wouldn't be difficult to add more lines and more bands, it's pretty simple circuitry.

    Mine is homebrew from a construction article by Chip Angle many years ago. But Russ W1VD at ARR sells this stuff, as do several others, all built and good to go.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Here's the 4-output sequencer from ARR: http://www.advancedreceiver.com/page20.html
     
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

  10. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    I have a pair of LANDWHER Preamps for 2 and 432 I need to install someday. they require a sequencer and special cable to the preamp (not a T-based power feed.) Some day I need to find the connectors to do so. I will really kick A$$ with the M2 33ft long 2M beam and that preamp in line. Have the antenna on the roof, just need to install it. Also have a 500ft reel of new LMR-400 to go to it.
     

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