2m mobile amp and 70cm

Discussion in 'Amateur Radio Amplifiers' started by N8SAN, Dec 3, 2015.

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  1. N8SAN

    N8SAN Ham Member QRZ Page

    Good morning all!

    I'm SURE this has been asked, but though my search I didn't catch anything. Most likely my searching.
    Anyway, should be an easy question.
    I have a 2m mobile amp, the old radio shack one. 30watts. My question is, if it is turned off, can I TX though it on other bands? Specifically 70cm? Or will the amp attenuate the signal too much?

    Thanks everyone!

  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Only way to know for sure is to try it and see -- doesn't cost anything and should take a minute.

    My experience with 2m amps made by KLM, Mirage, RF Concepts, TE Systems et al. is that none of them support 70cm operation well when they're turned off, because everything internally -- even in BYPASS -- is optimized for 2m and no other band; this includes the T-R relays and typical compensation circuits to adjust impedance back to 50 Ohms, compensating for the inductance through the relays and any other interconnecting wiring internally.

    But all you'd really need is a UHF wattmeter/SWR bridge connected between your dual band rig and the input to the amp to see if the amp changes the SWR compared with a direct connection to the antenna.
  3. N8SAN

    N8SAN Ham Member QRZ Page

    :Dcan't argue with that. I just wasn't sure if it would hurt the radio or even the amp. If the power from the radio wasn't allowed to radiate from the antenna that energy has to go somewhere....
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Oh, it will definitely pass through the amp and most will reach the antenna.

    However, there's likely to be some insertion loss as nothing in the amp is actually optimized for 70cm, including the relays and their connections; the input SWR which will be seen by your rig will likely be higher than if the amp was not in line, and the rig may cut back on transmitter power.

    But you shouldn't hurt anything.
  5. N8SAN

    N8SAN Ham Member QRZ Page

    Ok, so the amp attenuates the 70cm signal down to nothing. Never hear a repeater check in at all, granted 70cm isn't very active around here, but after months of scanning the repeaters, I've never head one. I also cannot make it to a repeater either. So I'm going to say you're correct, the radio is probably dropping its power output. It's a VX-8DR for reference.
    So in my minds eye, I should "T" the antenna, one leg would go to the ANT side of the amp, the other side will go to a switch.
    Then on the switch, on the "Y" side, I would have one leg to the other side of the "T" on the antenna, one to the TX part of the amp.
    Then the radio connected to the single side of the switch.

    My question is, if the antenna is just "T-ed" when I tx on 70cm, amp bypassed, power going to the antenna, would also be seen at the ANT side of the amp. Is this ok?

    Attached Files:

  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nah, that won't work worth a darn.

    What you need is either a DPDT coaxial bypass switch (ones that actually work on 70cm can be pretty expensive -- an "HF" inexpensive switch won't cut it), or two diplexers for 2m/70cm, which is the easiest way to do this that is actually guaranteed to work.

    Some common VHF-UHF diplexers (also called duplexers) are:

    W7ADC likes this.
  7. N8SAN

    N8SAN Ham Member QRZ Page

    oh well crap! "hihi"

    Thank you for those recommendations....
    Ya know, I feel silly now, as soon as I read diplexer I knew I should have known that!

    Thanks, very much for the research on the right parts.


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