Very frustrated with my HT / rooftop antenna. Any hints?

Discussion in 'General Technical Questions and Answers' started by KM6KCM, Jun 26, 2017.

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  1. K8JD

    K8JD Ham Member QRZ Page

    I see a couple of hilltop sites for several FM broadcast antennas (Several 50,000 Watt ERP transmitters) about 12 miles East of your QTH Maybe that is one of your problems
    Last edited: Jun 26, 2017
  2. AI3V

    AI3V Ham Member QRZ Page

    The FM radio coming in is the big clue for intermod.

    Minor im will do this, as the interference gets stronger, you will see stations "dissapear", and be replaced by silence.

    I have even seen top of the line yaecomwood ht's become completely deaf on all frequencies at tower sites.

    It is a artifact of the wide band nature of amateur VHF/UHF radios.

    Spending more $$ on another wideband rig is of little help.

    The solution is a bandpass filter, usually a "helical resonator" type .This type of filter is usually installed as standard equipment on commercial (public safety) equipment that only operates on a megahertz or so between "channels"

    Search "DCI bandpass filter" to see what the filter that will solve this issue looks like (and costs, they are a bit pricy)

    Also note that "better" coax or antenna usually makes intermod worse, not better.

    IMHO, intermod is the invisible reason that one ham reports a particular radio "hears" better than another at his qth, while another ham in a different location reports that radio is no good, and his other brand does better.

    Onthe service bench, testing with a single frequency signal generator, there is almost no difference in reception between brands, even baofeng vs Motorola, hook up a antenna, and put a hundred or more signals in a set is where the difference lays.

    A "quarter wave stub" type filter may help, but is less effective than the helical resonator type. It is also essentially free, just a coax "tee" and a few inches of coax.

    One note about filters, size matters! Bigger is better. (Less loss).

    N3AB likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    Biggest problem with most new "amateur" gear for VHF-UHF FM is that they are "general coverage" and receive everything from DC to daylight. Much more prone to intermod, desense, and other foibles.

    There have been "ham" rigs that didn't have this problem. The old Kenwood TR-7400A covered 144-148 MHz only, had a helical resonator front end and could not tune beyond the 2m ham band -- it had an absolutely amazing receiver, similar to commercial Motorola products. I bought one when they first came out in 1977 and I think there hasn't been another "2m FM" rig made that has a better and more "bulletproof" receiver than that had.

    I used to drive through "intermod alley" in NYC/NNJ, Philadelphia along the Surekill Expressway (!) and Boston through Needham Heights where all the VHF-UHF BC stations were, and while other hams always said, "Have to stand by, can't hear anything -- I'm driving through intermod alley," I never had that problem at all with the TR-7400A and had no idea what they were talking about.

    Pity it seems pretty much nobody is selling real "ham gear" that works in-band only (for reception) and has real serious front-end filtering anymore. Marketing won, engineering lost.
  4. VA7CPC

    VA7CPC Ham Member QRZ Page

    That was my first thought. You _know_ where the repeaters are -- it's easy enough to make up a "callsign vs heading" index card.

    The alternative (unless the repeaters are _really_ high, and _really_ close) is to get an omnidirectional gain antenna -- a collinear vertical, or something like that. It might be expensive, but you won't need a rotator.

    The nice thing about a gain antenna is that it helps you both on receive, and transmit.

    There's an old rule for HF, that you seem to have discovered works on VHF as well:

    . . . The better your antenna is, the better receiver you need, to avoid overload.

    . Charles
  5. KM6KCM

    KM6KCM Ham Member QRZ Page

    Thanks all for the tips -- lots to think about.
    1. Intermod is gone on the Kenwood TH-D72A. It was only a problem with the Baofeng.
    2. The coax run is ~90' between the roof mounting location and my study. I can cut 10' off of the run, no problem.
    3. Generally, things are much better since I was able to correct the weird Chirp misconfigurations. I'm hitting all of the repeaters I want to, and picking up repeaters that are significantly more than 15 miles away.

    I know that I'm asking a lot out of a HT, and do plan on upgrading to a 50w mobile at some point in the future.

    Thanks again all for the feedback!
  6. K6BRN

    K6BRN XML Subscriber QRZ Page

    10 extra feet of LMR-400 is not worth removing (almost no loss) - leave it in. And I'm not surprised that a Kenwood has better front end filtering than a Baofeng - you really do get what you pay for.

    One other note... you may be able to key up distant repeaters with 5 watts and a J-pole ... but ask for a signal quality check as you may not be driving the repeater to full quieting, which will mean that listening to your transmission will be painful for others and very annoying. If you are achieving full quieting you are "good to go".

    Have fun.

    Brian - K6BRN
  7. W0MSN

    W0MSN Ham Member QRZ Page

    Use that 10 extra feet of coax and raise your antenna.
    K9ZMD and KA9JLM like this.
  8. KM6KCM

    KM6KCM Ham Member QRZ Page

    Ha, wish I could.
  9. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I think you'd be much happier with a mobile rig at home than an HT.

    I used to live directly across I-80 from where you are - 3624 Jenny Lind Ave, North Highlands - I was not a ham at the time, but my dad was, and his two meter setup included a 1/4 wave spike at about 40 feet above ground. With that, he ran a net and took simplex checkins from all up and down the Big Valley from south of Merced to Grass Valley. So, the location does not suck, or at least didn't. I never have any trouble finding lots of repeater traffic when I'm in the area.

    A 50 watt radio will make a big difference.

    As others have pointed out, modern amateur grade FM rigs leave much to be desired in the receiver department. I've had a couple that howled in the night they were so bad in the intermod arena. I've had excellent luck with the Kenwood mobiles, however, and Yaesu also has some nice offerings, though I am less familiar with them.
  10. KM6KCM

    KM6KCM Ham Member QRZ Page

    A nicer rig is definitely in my wish list. For now, I'm limited to one radio. And for a few reasons, that radio needs to be an HT.

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