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¿ "Antenna to Load" ?????

Discussion in 'Ham Radio Discussions' started by KB3ZGV, Oct 21, 2012.

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  1. AF9J

    AF9J Ham Member QRZ Page


    Usually anything below an SWR of 2:1 is considered acceptable loading (the majority of the power - 66%, is being used up in the antenna system [antenna and feedline]), with the rest of the RF being kicked back/reflected back to the radio not being enough to harm the finals (with the exception of some older solid state radios). This does NOT necessarily mean the antenna is radiating the RF out acceptably ( as I mentioned before, the SWR your radio sees is for the antenna system [antenna, feedline, and [if you have one] antenna tuner/t-match, L-match,etc.], you could have a 1:1 SWR, and your antenna could be lousy radiator if the power is being dissipated in the feedline [G5RVs are notorious for having this issue in certain certain frequency ranges), but your rig won't have a fit (or at the least, SWR foldback protection circuitry, won't massively cut back the radio's output power.


    The tube finals in tube rigs are usually much more tolerant of mismatches (pre-WW2, hardly any consideration was even given to SWR - people just tuned for maximum RF current into the antenna system, and in many cases, the SWR was well over 5:1, yet tube finals were not used up left and right as a result of this). Most post WW2 tube rigs will easily shrug off a 3:1 SWR. Throw in the Pi-network used to tune/peak the tube finals, and most tube rigs will handle up to 5:1 SWR, since all they need is for the plate current of the finals to be within acceptable operating parameters/electrical load. Some of the tube rigs (like the Johnson Viking II I restored 3 years ago) have pi networks that will even load up (give the finals an acceptable electrical load), at an SWR high as 10:1 (this is due to the fact that when these radios were made, many amateurs did not have SWR meters, so they had to make sure the transmitter's finals, could be adjusted by the Pi-network, to a decent electrical setting [or at the very least, an electrical setting that wouldn't burn up the tubes], despite a lousy antenna system SWR).

    So basically, acceptable loading, depends upon your radio. Some radios (mostly solid state), are very picky about SWR, whereas some, kind of shrug it off )as long as it isn't extreme (above say 8:1 or 10:1). Adequate laoding typically means that the finals in your rig will see an electrical load/impedance that allows it to send a decent enough amount of power percentage-wise, into your antenna system, and out into (for lack of a better term) the ether.
     
    Last edited: Oct 21, 2012
  2. KB3ZGV

    KB3ZGV Ham Member QRZ Page

    So if the antenna is a poor radiator, and you still have a decent match, the power that is not radiated just becomes I^2R heating of the feedline?
     
  3. K7JBQ

    K7JBQ Moderator Volunteer Moderator QRZ Page

    No.

    Consider the dummy load. Perfect match. Terrible radiator (though contacts have been made with them).

    You're asking questions seeking "nutshell" answers. I suggest purchasing a couple of books and enjoying some quality reading time. I'd start with the ARRL Antenna Book and Reflections III by W2DU.

    73,
    Bill
     
  4. K7JEM

    K7JEM Ham Member QRZ Page

    Yes. The power has to go somewhere. Don't confuse "loading" with "efficiency". A dummy load has no radiation efficiency, but it will load well. A three inch dipole being used on 2M through 200 feet of RG58 will also load well, but be very inefficient.

    Joe
     
  5. K0RGR

    K0RGR XML Subscriber QRZ Page

    Some of the answers have touched on 'won't load'. It's a term that goes back to tube finals. With a typical output circuit, you had a 'Tune' and a 'Load' control. You would start with very light loading, and tune for a dip in the plate current to the final tube(s). Then, you would gradually increase the loading and redip the plate current. As long as the impedance of the antenna system was within the tuning range of the output circuit, as you increased the loading, the power output would increase. If the antenna did not match, the power would increase very little or not at all.

    It doesn't really mean much with today's solid state finals. 'My antenna tuner can't match it' would be the modern equivalent, and in most cases, the antenna tuner is performing the same function that the output circuit in the tube transmitter did.
     
  6. WA8LGM

    WA8LGM Ham Member QRZ Page

    Hey, some antennas hold their liquor better than others. That's why he can't get his antenna loaded!! :cool:
     
  7. G0GQK

    G0GQK Ham Member QRZ Page

    Its an old expression left over from the days when radio amateurs used valved transmitters, and the transmitter had to be loaded up properly to the antenna system. Things are much easier using tranceivers manufactured in the last 30 years, although some mutts still find it difficult

    Mel G0GQK
     
  8. WA4BRL

    WA4BRL Ham Member QRZ Page

    Having started amateur radio in the tube era, I am most comfortable with this answer. I'll add only the explanation that in this context, "load" means to couple RF to the antenna system. Increased loading therefore means increasing the RF energy (power) transferred into the feedline.
     
  9. W4HAY

    W4HAY Ham Member QRZ Page

    The term is relative. Yesterday I put up a new dipole and was checking the SWR. Heard a Morocco station on 20, called him with 50 Watts, and he gave me a 589. The SWR read greater than 6:1.

    A little later I heard Croatia on 10 and called. He returned a 559. The SWR was nudging infinity on the cross-needle meter. This antenna's not even supposed to work on 10.

    I'll prune the antenna tomorrow.

    When I got into ham radio in 1953, we didn't even have SWR meters. We cut the antenna to calculated length, put it up, and worked the world.

    First thing to remember: Don't sweat the small stuff. Second: It's all small stuff.
     
  10. W0NHH

    W0NHH Ham Member QRZ Page

    I got loaded last night. No antenna involved.
     
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