ANTENNA Tuner Correct Specs But Misleading

Discussion in 'Ham Radio Discussions' started by KC3EPA, Apr 15, 2018.

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  1. KC3EPA

    KC3EPA Ham Member QRZ Page

    I have a MFJ 1979 antenna tuner and it states it will accept an antenna impedance of 6 to 1600 ohms.
    Swr up to 32 to 1
    I guess 1600/ 50 is a 32 to 1 SWR

    I would think but my thought processes are hindered do to abuse.
    I use a 20 meter 1/4 wave on my mobile at 20 meters
    The unit will not match for 40 meters.

    I called MFJ and they said not going to match especially in mobile
    I was told a 20 meter mobile on 40 meters only presents about 4 ohms.

    Perhaps that is why the best it would do after clicking to find best car was 3 to 1
    It did match everything over 40 meters up to 10 meters but I could have done that adjusting the telescoping antenna.

    They did say it would not hurt to make sure the dip switches inside are correct and the interface cable was a little ratty but it should still find best match on transmit but the memory will not be set for recall as I was told.

    In any case I built a coil using 2 inch PVC with 24 tpi male on one side easily obtainable from home depot and a double female I had and used water putty to not move plus a end cap epoxied on.

    I used way too many turns but alligator clip takes me to 40 or 80 but 40 is what I was aiming for.

    These antenna tuners are great but not a miracle
    A manual one would have been nice but there is not much room in a Honda Accord

    MFJ does or is standing by the I terra e cable that looks like a 3 year old made it
    I was told most are crimped and my replacement will be soldiered do the copper being
    bare coming out of connector.

    What am I missing
    KB9BVN likes this.
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    You are missing the correct antenna for the job.
    A 20M antenna (No description, so, I will assume worst case scenario of a "Hamstick" type of antenna) on 40M "Should" show a low impedance. It also has low efficiency on 20M, due to it's construction.
    For your information, 40M mobile & down, usually requires a coil from the antenna to ground, to bring the impedance up to 50 ohms (WITH a somewhat resonant antenna).

    OK, now, for my "Assumption", based upon "Guesstimated" numbers, just to show you how inefficient your system is.

    Let's throw some numbers out.
    20M "Hamstick" style antenna mounted in perfect system (Placement & ground bonding). A generous 60% loss
    20M on 40M should generally cut that in half (In a perfect install, of which yours is not) So, another 50% loss.
    Using a tuner at the ends of it's range should be about an 80% loss.

    The coax, although short, I will assume RG58 with a 10:1 SWR, prettty much made your 100W radio into a QRP mobile, after all combined losses.

    AGAIN, none of these numbers are accurate. This is to try to show you that trying to make all of this work, for convenience, is a futile effort.

    W4IOA and ND6M like this.
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    Also, PROPERLY crimped connectors are usually as good, if not better than soldered connectors. At my job, we use about 4200 crimp connectors of 22 different kinds, in the past month.
    The only ones that fail are installed incorrectly, or abused beyond their limits.
    AND, this is hand crimping, with the proper (Sometimes up to $700) crimping tool.

    WC5P likes this.
  4. W4NNF

    W4NNF XML Subscriber QRZ Page

    Well, here's the thing. With a good enough tuner, I can tune up a lawn chair on 40-meters. I might even be able to make a couple of contacts. That doesn't mean it's going to be an efficient radiator. The antenna tuner allows the antenna to present a decent impedance to your transceiver so the finals are OK and the transmitter isn't folding back. Nothing more. What should you do, then? Get a good antenna, as good as mobile antennas get, for the band you want to use. ;)
    Last edited: Apr 15, 2018
    KA0USE likes this.
  5. NK7Z

    NK7Z Premium Subscriber QRZ Page

    Remember, even though your transmitter sees 1:1, (in a perfect world), the SWR on your antenna, never changes... The tuner just lets you inject power into the system. If the system is performing badly, (as a 40 meter mobile vertical will on 20), you still have a badly performing system, even if you see 1:1. You will just heat it up a bit more is all.
  6. KK5JY

    KK5JY Ham Member QRZ Page

    The 1979 is an antenna, not a tuner.

    Get an SG-237 for your mobile. It can match a 108" whip on 40m, 80m, and 160m if correctly installed. That 1979 whip will perform very well on 40m with an SG-237 or -230. I have done it many times. :cool:

    Also keep in mind that a whip by itself does not determine feed-point impedance in a mobile installation. The size, bonding, shape, and ground proximity of the car body underneath it also has a lot to do with the level of mismatch. The car body and the ground underneath it make up the other half of the antenna, and the installation details (as well as the vehicle details) cause a lot of variation between different installations.
    Last edited: Apr 15, 2018
    W2AI likes this.
  7. W0VRA

    W0VRA Ham Member QRZ Page

    And power out the antenna varies inversely with the square of the SWR of the antenna, right? So 100W into 2:1 radiates 25W, into 3:1 11W, and so on? A little friendly Elmering, please, no flames or telling me to Google or complaints about how the tests are too easy. :)
  8. KK5JY

    KK5JY Ham Member QRZ Page

    Well, not really.

    A mobile installation is just like any other vertical antenna installation. Ground losses, cable losses, SWR-induced cable losses due to reflections, cable length, etc., all contribute to efficiency. If you use lossless cable, you can get a 100% power transfer at the antenna with a desktop tuner on the dashboard, regardless of SWR. :cool:

    That's the reason remote couplers are helpful for mobile operation, because they put the matching network at the whip (if installed properly), which virtually eliminates losses in the feedline due to high reflection levels.

    And even with a remote tuner at the antenna, the ground losses tend to dominate in a mobile installation. My vehicles are all large trucks, so I get pretty good performance down to 40m... on a Dodge Dart, my expectations would have to be lower. ;)
    AG5CK and W0VRA like this.
  9. W0VRA

    W0VRA Ham Member QRZ Page

  10. AG5CK

    AG5CK Ham Member QRZ Page

    Most of the power can be radiated eventually (took me a while to understand that one) as long as feedline and tuner/matching device is kept to a minimum.

    This is a very informative video. It's long but I believe its worth the time. The good stuff starts at around the 6:40 mark.

    W0VRA likes this.

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