Mobile Questions

Discussion in 'Mobile Radio Systems' started by KG4HNY, Dec 10, 2012.

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  1. KA9MOT

    KA9MOT Ham Member QRZ Page

    OK. Let's assume since the radio is new, and un-moddified that the problem is not the radio. Let's assume that you know how to check SWR. I'd start looking for intermittent issues with the antenna system. From the rig to the ball at the top of the antenna. Make sure everything is well bonded (grounded) and double check the feedline.

    Can you beg, borrow or steal a Dummy load from one of your Ham neighbors?
     
  2. KG4HNY

    KG4HNY Ham Member QRZ Page

    I'll see if I can get a dummy load and report back. It will be tomorrow at the earliest.

    I will also do some more bonding and switch feed lines.
     
  3. KA9MOT

    KA9MOT Ham Member QRZ Page

    Sounds good. I'm sure by then somebody else will chime in with some more advice.
     
  4. KG4HNY

    KG4HNY Ham Member QRZ Page

    3 more bonding straps applied tonight, frame to body, bumper to frame. More coming when time allows. No improvement in SWR.

    Switched feed lines. No change.

    I was able to grab a dummy load tonight. It does not have a watt meter, unfortunately.

    USB into antenna = SWR light on radio at peak audio and choppy transmissions
    USB into dummy load = no swr light on radio

    AM into antenna = SWR light continual
    AM into dummy load = no swr light

    I rigged up a 10m mobile antenna into the same 3/8 x 24 connector. A local friend said I had no audio trouble on SSB or AM. He was able to hear me about 15 miles away.

    All this evidence keeps pointing me toward SWR problems. Don't flame me, but I have an auto tuner coming this week. Maybe I can integrate it and see if I can make some 40m or 20m contacts.
     
  5. K9ZMD

    K9ZMD Subscriber QRZ Page

    OK, Hamstick antenna instructions say something to the effect, do not force the metal rod deep into the fiberglass body because there is a wire inside that tube that the rod is likely to break. What's the history on your Hamstick antenna?

    If that earlier part did not set off any alarms, then how are you going about tuning your Hamstick for SWR reduction? I've read (but have not experienced) that if the exposed length of steel element is adjusted by simply sliding the excess inside the tube, then you are essentially creating a variable core inside the coils of wire on the antenna. Subsequent SWR readings will vary unpredictably as repositioning the "core" causes changes in the coil inductance. I say I haven't experienced problems like that because I've substituted heavy wire for the steel element while looking for the right length, never putting more than a few inches down into the fiberglass tube. Instead, I've started with an over-long piece of heavy wire (aluminum ground wire, on one occasion) and progressively trimmed the length until I found the SWR minimum. Once I have a good idea what length of exposed element is needed, I cut the steel element to the proper length, again allowing for just a few inches inside the fiberglass rod.

    Now, having said all that, I'm wondering if someone with vast Hamstick experience will come along and tell me that I've wasted my time because deep insertion of the metal element has no effect on antenna tuning. :)
     
  6. WN2C

    WN2C Ham Member QRZ Page

    I suggest you go back to k0bg's web site and re-read the info.
    My take is get a screwdriver antenna and get rid of the dummy load on a stick. Don't seem to be harsh, but that is... what it is
     
  7. KG4HNY

    KG4HNY Ham Member QRZ Page

    I read some posts about that too and really thought I was onto something. I am confident the whip has never gone very far down into the core. The excess whip length was trimmed and there was no change in SWR.

    I have read the website several times and continue to use it as a reference. The screwdriver style is best for radio performance, I'm sure. It will probably not be an option for my off road application but I will keep it in mind. That looks like a brittle piece of $350 waiting to snag a tree to me.

    Oh well... I still have just a little trial and error in me.
     
  8. K0RGR

    K0RGR Premium Subscriber QRZ Page

    You should be able to improve on that SWR, but using an outboard tuner is not a terrible idea. Your Hamstick or any fixed-tuned mobile antenna is only going to provide a great match over a small portion of the band. The higher the band, the greater the bandwidth of the Hamstick - typically. But on 20 meters, they tune fairly sharply, and on 40, very much so. The tuner should compensate, but you really want the antenna tuned for the portion of the band you want to use.

    Can you get hands on an SWR analyzer? They make it easier to find the actual resonant point for the antenna, and may tell you some other useful information.

    Since you are getting an SWR warning on voice peaks, it's possible that the radio is reacting to something it sees - particularly a lot of inductive reactance, of the kind found in heavily loaded short whip antennas. Remember that impedance is a function of both reactance and resistance. Ideally you want and antenna that is 50 ohms resistive with zero reactance, meaning it's resonant. Your SWR bridge may tell you that the SWR is OK, but what you're seeing is actually a strange combination of resistance and reactance that fools the SWR bridge but not the output circuits in the radio.

    There are ways to deal with this. The easiest thing to try for most people is to simply add a length of coax - 25 feet or more - in the line to the antenna. You may have to retune the antenna, as the apparent resonance will have shifted. In neolithic times, we would add transmitting capacitors across the base of the mobile antenna to tame it. MFJ makes a switchbox with selectable capacitors for this purpose. http://www.mfjenterprises.com/support/MFJ-909/MFJ-909.pdf Sometimes, the impedance at resonance is so low that you need a matching device at the antenna base. A tapped inductor is often used between the antenna base and ground. I had a terrible 'temporary' Hamstick set up years ago similar to yours, where I simply took a box of Radio Shack coax, and left it in line with the antenna. Very crude, but effective...

    I would also suggest you consider using the mount for the spare tire for your two meter antenna. I did this on a couple of my SUVs with outside tires. check the lug pattern for the mounting bolts, and get a wide piece of aluminum angle at the hardware store. I cut notches in each side and bent the angle iron to fit inside the spare tire, then added another piece of angle bracket over the open end. I drilled holes big enough for the lug bolts, and secured an antenna mount to the new 'frame'. Mine was more than strong enough to hold Hamsticks and Hustlers for HF, too, but I used it for VHF/UHF - the added height helped.
     
  9. KG4HNY

    KG4HNY Ham Member QRZ Page

    Auto tuner fixed the transmission problem. Thanks to everyone for all the great info in detailed responses. I am still trying to perfect this setup.
     
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