EFHW Antenna: A Detailed Analysis (NEC4.2)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9XMT, Mar 29, 2018.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd think the UK transmit power levels may be part of that. RFI issues with unbalanced antennas tend to get worse as transmit power rises.

    FWIW, I've never had noticeable RFI issues with end fed antennas at QRP levels, have had mixed results at 100 watts but have dealt with a lot of RFI over 600 watts or so. So when I see posts on these forums or elsewhere where a ham is struggling with RFI with certain unbalanced antenna installations and is running 1000w to 1500w I'm not really surprised.
     
  2. W9XMT

    W9XMT Ham Member QRZ Page

     
    Last edited: Apr 2, 2018
    KD6RF likes this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    We only have one Mains feed in domestic properties, which is 240V.

    But our Earth cables around the house have to be pretty substantial, and all our power circuits are wired in a loop/daisy chain, so that a feed (including the Earth wire) comes to and from the fusebox in both directions. In addition, every radiator has an earth cable that connects to the earth wire in the nearest power socket in that room. Even the incoming metal gas pipe has to have a substantial cable connecting it to the Mains Earth point. So all in all, there is usually a huge framework of copper pipes and copper cables all over the house, providing a large amount of "counterpoise", as well as being quite well grounded via the metal pipes arriving underground into the building. (it would certainly be impossible to model on a computer programme)

    However . . . we never have ANY kind of lightning arrestors etc on any electrical or antenna cables coming into the house. (lightning strikes are extremely rare here in Britain)

    Regarding power levels . . . I would imagine that the percentage of people with Linear Amplifiers is much the same in the the USA as in Britain. (My guess would be about 10 per cent)

    And those running Amps are likely to have the same range of power output as in the USA !

    Regarding Trips . . . most modern fuseboxes have an RCD - residual current detector. That detects any slight inbalance between the Live and Neutral power cables and trips . . . as any inbalance suggests a current leaking to earth. This is there mainly to cut the mains if a person is providing that earth leakage (ie to prevent death from electric shock) . . . but in practice they trip if the heater element in an electric kettle, washing machine, etc is starting to leak slightly inside.

    However, I've never known these trip due to RF !

    Regarding Fuses, my house is quite new, but it has conventional cartridge fuses for each of the separate power and lighting circuits. (some houses have re-settable trips for each circuit . . . but again I've never heard of people having trouble with these tripping due to RF)

    Roger G3YRO.
     
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  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I was under the impression that maximum RF transmit power in the UK was 400 watts, not the 1500 watts allowed by the FCC. Has that changed or is exceeding the legal limit a common thing in the UK?
     
  5. G3YRO

    G3YRO Ham Member QRZ Page

    That's like asking if everyone sticks to our 70 mph speed limit ! Of course they don't.

    To use a famous quotation, my attitude about such things is that "Rules are for the guidance of wise men, and the blind obedience of fools."

    I would imagine that most people that buy a 600watt amplifier run 600 watts output . . .

    But why would someone waste money on a bigger Amp, if they didn't intend to sometimes run it at it's rated power output?! (and continuing the above analogy, why would anyone buy a powerful sports car if they always stuck to the speed limit?)

    I think that most Amateurs over here - especially those who have been around a long time, and passed the original quite difficult technical exam to get their licence - think that our 400 watt output limit is unecessarily low. It's about running power WHEN NECESSARY, and using it responsibly. (ie making sure your signal is clean, and that you're not causing local domestic RFI)

    I've always been a keen 160m DXer . . . but for years, our limit anywhere in the band was 10 watts INPUT ! Well when I first got my licence when I was 14 I actually worked across the pond (W1BB) running a genuine 10w . . . but soon realised that all the British DXers were running a lot more power than that !

    Let me turn it around - do you think people in the USA who own a 5kW Amp stick to 1500 watts output?

    Roger G3YRO
     
    Last edited: Apr 3, 2018
  6. 2E0VSS

    2E0VSS Ham Member QRZ Page

    Power..It’s a bit like a divorce settlement, one is always trying to squeeze out that last tiny drop and obliterate the foe..
    2e0vss
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Good point.

    Of course the difference being the commercial amp market has a lot more amps in the 600-1500 watt range which are legal here and you have to look a bit harder to find a 5kW amp. But yeah, human nature being what it is power levels may not be all that different regardless of what Ofcom says.
     
  8. G3YRO

    G3YRO Ham Member QRZ Page

    So coming back to using this type of end-fed half wave . . .

    I agree it's POSSIBLE that you might get RFI issues (even though I never did), due to the fact that you are on the third floor and the grounding is a from big network of cables and pipes that only eventually connect to earth . . .

    But what about the EFFICIENCY of such a system?

    I always figured that because the antenna feedpoint was very high impedance, and no feeder as such (just a couple of feet of coax from the ATU to the Tx/Amp) there would be minimal losses.

    Over the years I have had 4 QTHs where I had end-fed 160m Half Waves, and 5 where I had Half Wave Dipoles (centre fed with coax, in the usual way), including my current QTH.

    Certainly it always seemed that the End Feds worked just as well as the Dipoles.

    Roger G3YRO
     
  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    I think that using the word "efficiency" when discussing antennas leads to all manner of misunderstandings.

    Sure there is a pretty clear IEEE definition, but this is not what most hams mean when they say "efficiency". After all most of them are not antenna engineers.

    I think what most hams are trying to say when they use the word "efficiency" when talking about antennas, is a much more vague word like "effectiveness". But they don't want to sound wishy-washy so they say "efficiency" thinking it is more technical.

    How well does the antenna "work" after all?

    "antenna working" often is not defined using meaningful engineering terms like gain, pattern, sidelobes, return loss, bandwidth, far field intensity and so forth.

    An antenna with poor efficiency is not likely to be a good performer. However, there are lots of antennas with good efficiency that are not good performers, or do not meet with the users operational objectives. For example, just because an antenna radiates all of the energy applied to it (100% efficient) does not mean that radiation goes in the direction you want it to go.

    These endfed-half wave type antennas seem deceptively simple. But the unspoken complexity, variability and sensitivity to small changes is why so many users have different experiences.

    Even if a specific antenna is performing adequately as a transmit antenna, it may be an inferior receiving antenna. Times are changing and with this change has come a cesspool of RFI in many locations. Antennas where the feedpoint or other sensitive sections are in proximity to interference sources are likely to be susceptible to this kind of interference for many users. These Endfed Halfwave antennas are often used in this fashion and some users are dissatisfied. If you are on the trail in the middle of wilderness, it may work fine. Near ground clutter, buildings, interference sources, conductive objects that provide unknown coupling and shunting of RF currents may give different results.

    These are some of the reasons that I think other simple antennas are a better starting solution for amateur usage. Not because the endfed halfwave can't work, but that it is unpredictable for many casual users.
     
    Last edited: Apr 3, 2018
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  10. KD6RF

    KD6RF XML Subscriber QRZ Page

    Well, no. Not necessarily. Back to all of the data posted earlier - we have the COUNTERPOISE., which in the case mentioned above (where a short length of coax is connected from transformer to transceiver) the coax + the house wiring all the way back to ground and power lines IS the counterpoise.

    And as shown earlier, small current at the feedpoint does NOT mean small current elsewhere. Nor does it mean small LOSSES.

    [​IMG]


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    I guess it must seem counter-intuitive, but it just keeps popping up time and again ----- We have to remember that ===> "Just because the current is low at the feedpoint, does NOT mean it is low everywhere else"...

    The current is higher on down the radiator wire, and it's often higher on down the counterpoise. Just how much higher or lower depends upon the details of installation. And whether or not it causes RFI, inefficiency, or noise pickup also depends upon the details of installation.
     

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