Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA6MHZ, Jan 4, 2012.

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  1. WA5BEN

    WA5BEN Ham Member QRZ Page

    That indicates that you are mis-using the term "resonant".
    The important thing to look for with an analyzer is the frequency at which both XL and XC are at ZERO. That is the resonant frequency.

    From your description of the installation, that should be at an input impedance of around 10 to 14 Ohms, which will present a VSWR of somewhere between 5 (for 10 Ohms) and 3.5 (for 14 Ohms). All 3.5 MHz 8 foot antennas similarly mounted will have input impedance close to that range (with the best being LOWER impedance -- because losses will have been minimized), and all require matching to a 50 Ohm transmitter. That match can be as simple as a tapped coil at the base of the antenna, or as complex as an autotuner like the AH-4.

    At 10 Ohms input impedance, you are actually capable of radiating a maximum of 10% (10 Watts) from your 100 Watt transmitter. At 14 Ohms, you are capable of radiating only 7% (7 Watts), and at 20 Ohms (fairly typical of 8 foot antenna on the bumper or trailer hitch, with a small and lossy coil), you are down to 5 Watts.

    All HF mobile antennas require impedance matching. You cannot simply connect them and tune for lowest VSWR. Tune the antenna for resonance, then tune the matching network for impedance match to the transmitter. If you don't want to play with those slightly interactive settings (and be limited to 10 kHz of bandwidth on 75), get a REAL autotuner -- i.e., one that mounts at the base of the antenna (in my case, inside the trunk of my Sebring convertible or on the roof rack of our Expedition). The Sebring carries the antenna on the left rear deck, using a home-brew stainless steel mount. The Expedition flies the antenna on the roof rack, again on the left side, and also using a home-brew mount that employs a commercial mount as one element, two stainless steel u-bolts, stainless washers and nuts, and stainless anti-vibration nuts as the "shake-loose-stopper".
  2. WA5BEN

    WA5BEN Ham Member QRZ Page

    Just tell him my phonetics: "Wonderful After 5 Better Every Night" :)
  3. K4KWH

    K4KWH Guest

    I used to run into many "Extra Class" hams that came to my shop complaining of HF antennas that "didn't work". I immediately went right to the mount and found that the Impedance device was mis-tuned, OR simply not there. Many would-be hams have no understanding of what you've said here, and complain mightily about the manufacturer of their antenna when it is, most often, THEIR own fault. It's not "manly" to read instructions, I guess! :) When I would ASK where the instruction booklet was, many times the answer was, "I lost it". Really? Accident or on purpose? It's why I quit making antennas for anybody but ME! :( Let 'em swim in their own pond! LOL!

  4. W1GUH

    W1GUH Ham Member QRZ Page

    Had the same experience, kinda. I've got generic hamsticks for 160-10. The ones called "HF Power Rod" have all been good. Tuned and got out fine. The other brand I've gotten, for 15 and 17 aren't. The top part of the 15m stick needed about an inch cut off of it to get the antenna to tune. The 17m stick is unusable. I cut off a good 4" & it was up to about 16 Mc. Garbage! I'll look up the brand of those next time I'm in the car.
  5. K1DNR

    K1DNR Ham Member QRZ Page

    No. Tune for minimum SWR to minimize loss in the feed between the antenna and the transmitter.

    There is no such generalization. It depends on many factors including the antenna, the mount, the design, the manufacture, etc, etc... Lower impedance doesn't minimize anything. Low SWR minimize loss in the transmission line, that's it. If anything, a higher feedpoint resistance means a more efficient radiator, not a less efficient one.
    I'm totally not following your logic. The impedance has ZERO to do with how much power is radiating. Q of the loading section determines how much power is lost.
    A Hamstick designed for a particular band should be able to be tuned for lowest SWR. If not, there is something wrong with the mount, the location, or the hamstick itself.

    A Hamstick for 80/40/whatever band is designed to provide a minimum SWR at some point on that band WITHOUT a "Real" tuner. That's the whole point of using a Hamstick... Tuning something that already contains a matching network is kind of silly... unless perhaps you are trying to extend the BW from the CW into Phone portion of the band or vice versa.
  6. K1DNR

    K1DNR Ham Member QRZ Page

    Let me make a correction. The coil isn't the only point of loss in the antenna.

    I still fail to see how one can determine the percent radiated power based on the R value of the feedpoint impedance. You need to know the radiation resistance of the antenna itself.

    Either way, any 8 foot antenna is going to be inefficient - even if it "works" - on 80m.

    By design, the antenna should reach an acceptable SWR without a tuner at some point in the 80m band - which is of course quite wide.

    The manual suggests that filing of the stainless whip may be necessary in some cases.
    Last edited: Jan 11, 2012
  7. KF5FEI

    KF5FEI Ham Member QRZ Page

    Sounds like our customers. They call us with an install question that is covered in the instructions, which are rubber-banded to the product and sealed in a bag when they leave here. When asked if they have the instructions, they claim they never got them.
  8. KA4EET

    KA4EET Ham Member QRZ Page

    Not sure I see what you are trying to do with the so-called ham Stick for mobile operation. First of all, you really need the hamstick to be "made" for or be resonant on the Frequency you are wanting to work. I have delt with many mobile whips and I find the old Hustler whip (Screw top resonators) with the "tri-pod" mount is about the closest thing you can get to be useful on more than 1 frequency.(Unless you have the power whips that you can adjust the antenna size by push button) I have hamsticks here that I have mounted on my pool cage and also used them as rotatable dipoles on my short tower. Very good perforamance with the ones that are 20 meters and higher. Crappy performance with the ones on 40 and 80. The physical size in relationship to the band seems to be real important with the performance.
  9. K4KWH

    K4KWH Guest

    A Hamstick for 80/40/whatever band is designed to provide a minimum SWR at some point on that band WITHOUT a "Real" tuner. That's the whole point of using a Hamstick... Tuning something that already contains a matching network is kind of silly... unless perhaps you are trying to extend the BW from the CW into Phone portion of the band or vice versa. (quote)

    That's true. A 75 Meter hamstick takes care of part of the total system impedance with the lossy loading coil. It should not need matching in and of itself. It is a fact of a hamstick's sloppy, lossy characteristics that this is true. This is not to disparage those who like/use hamsticks for whatever reason. I opt to be HEARD with high-as-possible-efficiency mobile antennas, and, for ME, it means either some sort of Bug Catcher, or a large diameter screwdriver with at least #14 wire and wide spacing. One of the advantages I love with the screwdriver style (Thank You! Don Johnson!) is the fact that you can tune on the fly. A mobile antenna traveling thru differing environments encounters varying "RF" obstacles that throws a HI Q HF antenna off. You can have your fixed-tuned Hi Q antenna tuned to a "T", and while driving down the road in a rare QSO, a big truck pulls alongside, and spoils everything! DRAT! :( A screwdriver? A couple bumps on the switch, yer back in business. This is what, IMHO, made HF mobile popular again, and why I think they are best in overall mobile efficiency. Fewer interruptions, more time actually communicating, and full flexibility thru out the HF range! I would also point out that there is a point where a HUGE Bugcatcher is at a disadvantage and it own hi efficiency and narrow bandwidth makes it less tolerant of the fluid environment thru which it travels. This is where the screwdriver of almost any size outshines fixed tuned mobiles. Yeah, yeah, I know. Them HF "antenners" are big and ugly, but it is what it TAKES to get the job done, IMHO! :p
  10. K1DNR

    K1DNR Ham Member QRZ Page

    A good example.

    I didn't get all the theory points correct. Actually, I believe we'd have to look at a Hamstick as a 4' antenna (or however long the stainless part is). We'd have to calculate the actual efficiency of that Hamstick on 80/75m. We'd need to know what part of the feedpoint R is actual losses and what part is radiation resistance. The loading part is a loading part. It is not a helically wound antenna like a Firestick. So the actual base of the antenna is the stainless whip. The loading portion is just the loading portion - to bring the feed point closer to 50+j0.

    The generalizations are still incorrect.

    Either way, a Hamstick is a really awful antenna wrt efficiency on 80/75m.

    And yes, the lossy loading coil is part of the design. Kind of half dummy load and half antenna - but it should still present a match somehwere on the band if tuned and mounted properly. Otherwise something is wrong.

    Also assuming the analyzer is being used at the feed point of the antenna and not through some length of transmission line.

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