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Non-Resonant Antennas

Discussion in 'Discussions, Opinions & Editorials' started by KL7AJ, May 9, 2007.

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  1. KL7AJ

    KL7AJ Ham Member

    It's really amazing how persistent so many bad "theories" are, no matter HOW many times you keep reminding them. The same misconceptions pop up here on QRZ time after time after time after time after time after time after time......

    Probably the most persistent of these is the proclamation that a resonant antenna is better than a non-resonant one. This fallacy has been propagated since at least as far back as 1956, when Byron Goodman, W1DX was compelled to set the record straight in his classic article "My Feedline Tunes My Antenna" (Attached) And yet, there were two posts on QRZ just today by (unnamed) people who should definitely know better.

    It is the RESISTIVE component of any antenna that determines the radiation. REACTANCE has no effect whatsoever. ALL A.M. broacast towers are non-resonant. Every one of them. Not by choice, but by the fact that nobody cuts a broadcast tower to length.

    You don't need a broadcast tower to prove this. It can be demonstrated with any wire antenna, a field strength meter and a tuner. There should never be any reason for this misinformation to keep coming up. But I'm sure it will...QST has had to republish W1DX's paper countless times.

    Now...if I could just get paid...... [​IMG]
     
  2. WA4ILH

    WA4ILH Ham Member

    And, ... your coax HAS to be cut for an exact 1/4 length or it wont work. (10-4?)
    Tom WA4ILH
     
  3. WB2WIK

    WB2WIK Premium Subscriber

    Resonance hasn't any magic, but certain resonant antennas will match common transmission lines (coaxial cable) well without the need for other devices, and that is the magic.

    A 1/2-wave center-fed dipole and a 1/2-wave end-fed wire are both resonant and have identical radiation patterns. Or, they would if only you could match the end-fed wire effectively to minimize common mode currents and prevent the feedline from becoming part of the antenna. That's not so easy!

    If you need to convince somebody there's no magic to resonant antennas, discuss the most popular VHF whip antennas, which are all 5/8-wavelengths and non-resonant on their own. They're fed through an inductor that cancels the Xc of the whip and makes them resonant, though.

    Ditto the broadcast towers. They're selected for a particular height based on operating frequency, the ground beneath them and the radiation pattern required to maximize ground wave coverage while minimizing co-channel and adjacent channel interference, usually without regard to sky wave since that's not the primary audience nor protectorate. Then, whatever length that turns out to be, they're tuned to resonance with a matching network. The system becomes resonant; the radiator doesn't need to be.

    But I like current-fed resonant antennas because they make my life easier and allow rapid deployment of effective antennas with predictable radiation patterns.

    That's not a bad thing.

    WB2WIK/6
     
  4. KE5FRF

    KE5FRF Ham Member

    Thanks you guys for presenting information on these topics. I've learned a lot from some of you guys and implement your knowledge wherever I can, but most importantly, I try to make real world comparisons and base my final judgements from there.

    I think Steve makes a fantastic point that for certain antennas, especially those designed to be resonant on a particular band and where a predictable radiation pattern is desired, coax is best.

    But I also realize that 75% or more of hams have property limitations as well as budgetary restraints that make the ideal impractical. Steve tells the ideal truth, which is important.

    Eric presents the story for the common ham. The ham who has to live with compromises. For the ham who lives with compromises but desires frequency agility, balanced line fed multiband antennas are "the only" way to go.

    No magic, voodoo, or old ham's wives tales. I've seen the difference in my short time. Coax feeding an antenna and trying to tune it on various bands simply doesn't work.
     
  5. WZ4I

    WZ4I Ham Member

    My non-resonant 260ft dipole works excellent fed with 450 ohm ladder line. I've compared this antenna to a 1/2 wave 40m dipole I had up, but not quite as high, and the 260ft dipole was always 1 to 3 S units better on xmit, and slightly better on receive on 40 meters....

    73 and good DX
    WZ4I - Mark Harrison
     
  6. AG3Y

    AG3Y Ham Member

    Well, I just modified my 20 meter full-wave vertical loop today by attaching a half wave long piece of ladder line to the feedpoint, and moving the 1:1 balun to the other end of that line. As far as I can tell, the performance did not change on 20 meters, if anything it actually is somewhat better since the currents are being balanced a bit better by the ladder line feed ( ? ).

    The big surprise is that the 2nd station that I worked on 40 tonight was located in the United Kingdom ! ! ! The 1st station, which was about 90 miles away from me, was running a kilowatt and I was barefoot. Station #1 and I were still in our QSO when the English station jumped in to the "group". He gave Nick an "S-9 +10 " report, and I got an "S-9". I was fairly blown away by that as I could not even load up the loop on 40 prior to adding that quarter-wave matching link! So, as has been said above, the antenna itself is not resonant, but the system presents a 50 ohm resistive load to the rig, which is what it wants to see!

    I will have more to report about this antenna in days to come, I am sure. But it sure seems to be doing great on its "maiden voyage" !

    73, Jim
     
  7. NZ3M

    NZ3M XML Subscriber

    So, if I tune my 6m beam on 160m, it will work just as good as a 260ft wire?

    Wow, thanks!

    [​IMG]
     
  8. AG3Y

    AG3Y Ham Member

    No No NO ! There is a little thing called "radiation resistance" that comes into play here. A short whip that is tuned to resonance with lumped components ( coil and capacitor stuff ) will probably only have a radiation resistance of a few ohms, while the entire system still presents a 50 ohm resistive load to the transmitter. In order to push even 100 watts of RF out of a system with only a few ohms of radiation resistance, the system would have to support a current flow of many many amperes! ( P = Isquared*R ) It is no wonder that a few feet of wire will not radiate worth anything on the HF bands, even though many a mobile station has tried to put out a reasonable signal on the lower bands and failed miserably. Low radiation resistance, no RF grounds, etc. etc.

    Read the book ! 73, Jim
     
  9. KJ3N

    KJ3N Ham Member

    Really?

    Then I guess this little guy working The Netherlands, Germany, and Portugal on 75m is just a figment of my imagination? [​IMG]

    Thanks for clearing that up. [​IMG] [​IMG] [​IMG]
     
  10. K9ZMD

    K9ZMD Subscriber

    Doesn't matter if it's a weak transmitter, or a weak antenna, QRP is QRP. It works, it's fun, and its success often depends on the distant-end operator's skill, patience, and antenna.
     
  11. W1VT

    W1VT Ham Member

    Another example of a non-resonant antenna is a Yagi with a hairpin match. The driven element is shortened to make it capacitive--resonating it with an inductor steps up the impedance to obtain the desired impedance match.
     
  12. WB2WIK

    WB2WIK Premium Subscriber

    Why is a 260' dipole non-resonant?

    A 260' dipole is resonant at 1.8, 3.6, 5.4, 7.2, 9.0, 10.8, 12.6, 14.4, 16.2, 18.0, 19.8, 21.6, 23.4, 25.2, 27.0 and 28.8 MHz. Seven of those frequencies fall right smack inside amateur HF band allocations; three more fall very close.

    I'd say a 260' dipole is very much resonant on 160, 80, 60, 40, 20, 17 and 10 meters, and just slightly off resonance on 30, 15 and 12 meters.

    It's current-fed on 160 and 60m and voltage-fed on the other bands; but it's still resonant.

    WB2WIK/6
     
  13. WA9SVD

    WA9SVD Ham Member

    I wouldn't expect the performance on 20 to change much just by adding a half wavelength of ladderline, and the balance probably isn't any better than it was with the balun in it's original position. (Unless the ladderline is well away from metal objects or the ground, it could actually upset the balance.)
    =================

    "...the system presents a 50 ohm resistive load to the rig, which is what it wants to see!"
    =================
    Presenting a 50 Ohm resistive load is what the transmitter wants to see; that's true. That way, the trensmitter will put out maximum power. But that's no guarantee that the antenna system will be an efficient radiator. Remember, a dummy load is also a "50 Ohm resistive load," but makes a poor antenna.
    Providing the transmitter with a resistive load is only half the battle; the rest of the task is to use an efficient radiating system, whatever form it takes.
     
  14. AG3Y

    AG3Y Ham Member

    Note: I DID say "many a mobile station", not ALL of them! There IS a difference, you know!

    I worked mobile some years ago using a simple whip on 15 meters. On one memorable afternoon, I parked my car in a lot and worked 5 different countries with 5 different transmissions!

    But that was an exception, rather than the rule, and I was on 15 during high sunspot activity, not 160 during a low period !

    I stand by what I said.

    73, Jim
     
  15. AG3Y

    AG3Y Ham Member

    SVD, agree with all you are saying. A half-wave loop on 40 meters has a lot lower radiation resistance than a full wave one would have, so it's efficiency is bound to be lower ( I'll let you figure it out, I don't have an antenna modeler ), but it is a darn site higher than a "magnetic loop" only a few feet in diameter! As they say, "it's all relative". As I said earlier, this is an experiment in progress. I expect that it will work better some days, and worse on others. I will let you know down the line.

    73, Jim
     
  16. KJ3N

    KJ3N Ham Member

    As my mobile setup proves nearly every time I use it. [​IMG]

    First of all, I'm yanking your chain, Jim. Chill.

    Second of all, a HamStick on 15m isn't all that inefficient in the first place, considering that a 1/4 wave for that band is about 11 feet. A HamStick on 75m is a another thing all together. I wouldn't expect much in the way of results using a HamStick on 75m.

    Well, you're certainly welcome to.

    However, I would have to say that (in general) if you take the time to do your mobile setup correctly, you won't "fail miserably" when it comes to the lower bands like 40m and 75m.

    Those 3 countries on 75m I've mentioned were done within the last 9 months or so. On 20m, I've worked enough stations on either the first or second call to tell me that I'm not "failing miserably". I don't spend enough time on 40m from the car to gauge how well I'm doing there. Obviously, YMMV.

    As to 160m mobile, I'm not going to touch that one. That's too much effort, even for me. [​IMG]
     
  17. AG3Y

    AG3Y Ham Member

    3n, I've actually got goose bumps ! Not a problem ! [​IMG]

    BTW, I've always loved your avatar . 73, Jim
     
  18. K0RGR

    K0RGR Premium Subscriber

    A fullwave loop will have an impedance of about 100 ohms at resonance. If it's less than a halfwave above ground, the impedance will be lower - closer to 50 ohms. That loop will also have usable impedances at every harmonic of the resonant frequency, so a loop that resonates at 7.1 Mhz. will also resonate at 14.2, 21.3 and 28.4 Mhz.. It works great on those 4 bands when fed with coax.

    But, if you feed the same loop with open-wire feeders and a tuner, it will work fine on all frequencies from somewhat below 7 Mhz. up.
     
  19. WA9SVD

    WA9SVD Ham Member

    I didn't mean to argue or "dis." Your current system (as you say) still works well on 20. I'd expect that. The ladderline feedline is probably adding to the radiating portion on 40 Meters, which is why you can get it to tune on 40 now. That's a GOOD thing, as long as you don't have stray RF floating around, or cause TVI to the neighbors.
    Even after all the theory and modeling, it's still a "put it up and try it" world out there. Glad your system is working for you.
    I just wanted to point out that just because a system presents (or seems to present) a 50 Ohm resistive load TO THE TRANSMITTER is no guarantee it will be an efficient radiator.
     
  20. AG3Y

    AG3Y Ham Member

    I doubt that the feedline is radiating, because it is being fed with a balun, is feeding into a balanced load, and is supported on a wooden structure a good distance off the ground.  It is a simple case of impedance transformation, because the line is 1/2 wavelength on 20 and only a quarter wavelength on 40.  

    BTW Trying this out was not my original idea. It came out of QST from just a few months ago.

    73, Jim
     
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