Antenna Modeling of SOTA/POTA (20M EFHW)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KM1NDY, Apr 11, 2021.

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  1. KM1NDY

    KM1NDY Ham Member QRZ Page

    Hi:
    Enjoy and/or criticize this blog post. It is a fairly comprehensive look at the propagation pattern of yesterday's SOTA/POTA of Mt Wachusett in Princeton MA from my current level of understanding. As always, feedback is welcome.

    https://km1ndy.com/antenna-modeling-mount-wachusett-sota/

    Best,
    KM1NDY
     
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  2. K9UR

    K9UR Subscriber QRZ Page

    Good blog. I suspect that radiowaves end fed half wave uses a 49:1 unun or perhaps a 64:1 unun. A 9:1 unun would not give an adequate transformation for a half-wave antenna (due to high impedance of approx 2500 ohms). A 49:1 would give an impedance transformation much closer to 50 ohms that our coax cable and radios like to see.

    The issue you're seeing with regard to pattern is likely due to the height above terrain. You will get ground skew if you are not at least 1/2 wavelength (10 meters, or approx 34 feet) in this case. patterns improves even more if you can get 1 full wavelength in height above terrain, ie, 20 meters high or approx 66 ft. Other factors that can change the pattern is the orientation of the feedline since the end fed antenna is known to suffer from common mode currents on the outside edge of the shield of the coax cable. This can change antenna patterns and even give vertical radiation to an otherwise horizontal antenna. One solution is the use of a "rf choke" a few feet below the antenna connection point to the coax, along the coax cable.

    As for range / etc, single hop on 20M via the F1 and/or more likely the F2 layer during daytime hours in mid spring (considering this time in the sunspot cycle) isn't surprising. Paths are typically 1000 miles or so ...with a few multiple bounces allowing for 3000-4000 kms range. This is right in line with the modeled results.
     
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  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Some suggestions about modeling your antenna using EzNec.

    If at all possible, avoid the use of the "MiniNec Ground" model with a low-mounted (< ~ 1/4wl above ground) horizontal wires. Use the "High Accuracy" model instead.

    This antenna is designed to be fed through a 1:7 turns ratio (50:2450 Ohms ratio) transformer.

    You are using too few wire segments to create an accurate model. You are likely using the "free" version which is limited to 20 segs. An accurate model (one which passes the "Average Gain Test" see the Help File) likely requires more than 35 segs, and multiple wires.

    Your placement of the "transformer" (actually its 2450 Ohm winding) in the antenna's radiating wire is simplistic. In the real antenna, the coax shield running all the way to the radio acts as the part of the antenna. I acts like the "short" wire on the "other" side of your source. You modeled it as 1.5 segs of the main wire, while in the real world, it is whatever the length of the coax shield is...

    The correct way to model this antenna is to use four wires (not 1). Wire1 is the antenna wire with lots of segments. Wire 2 is a short continuation of Wire 1 with only three wire segments used as a place holder for the transformer secondary winding. Wire 3 + 4 represents the path followed the coax shield from the transformer to the radio.

    To model the impedance transformations caused by the transformer and the coax, use Virtual wires (see the EzNec Help file).

    Connect the "source" to V1 (Virtual node 1). Connect 50ft of RG-8X (or whatever you are using) from V1 to V2 (virtual node 2). Connect the 50:2450 Ohm transformer from V2 to the seg 2 of a short wire #2. Create the path representing the coax shield as Wires 3&4,... as many wire pieces as are needed to reflect the coax routing.

    I am attaching a model which is closer to your reality. This model was created using AutoEz, but I am posting the EzNec file created by that process to show how to model the coax shield and its effect on radiation and feedpoint impedance, the method of accounting for the impedance transformation caused by 50ft of RG8X (including accounting for the Velocity Factor and loss of the coax), the method of accounting for the impedance transformation caused by a 1:7 turns ratio transformer, and the common-mode current flowing on the outside of the coax between the transformer and radio.

    To do justice to this reality, you should be using a minimum of ~33 segments. If I reduce the total number to only 20 so that the model can run on the free, demo version of EzNec, the model looses accuracy per the AGT test.

    I am only putting 20 segs in the model I am posting for you. When I run that model with only 20 segs, EzNec complains that wire segments are too long. This could be fixed by running the paid-for version and increasing the segment counts...

    The model predicts a feedpoint impedance of 39 + j30 Ohms, for a swr50 (as seen by the transmitter) of >2. This could be reduced by putting a coax CM choke (1:1 unun) on the vertical drop of coax, about 25% of the way down the vertical coax section (~4ft below the transformer). That drops the swr at the transmitter to less than 1.5. I have added the choke to EzNec model; you can run the model with and without the choke to see how it effects current distribution on the antenna wires and how it changes the feedpoint impedance.

    I went back to AutoEz, and optimized this antenna to "improve" it at the center of the 20m band (14.175MHz). To run the optimizer, I increased the wire segmentation. I will let the optimizer vary the wire length, the transformer turns ratio, the distance from the transformer along the coax to the coax CM choke (1:1 unun), all in an attempt to improve the Swr at mid-band. The placement of the antenna is as you specified; transformer at 15ft agl and end at 20ft agl. This would change if the antenna was mounted higher relative to dirt.

    After optimization, the feedpoint impedance is 51.8 - j3 Ohms for a swr50 = 1.06 at 14.175MHz provided the main wire is insulated #14 awg wire, is 33.4ft long, the transformer is changed to be 1:8 (50:3200), the coax CM choke is moved to 6ft below the transformer.

    You might learn something about EFHW antennas by reading a slide show I created. It is linked to at the bottom of my QRZ page.
     

    Attached Files:

    Last edited: Apr 11, 2021
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  4. KM1NDY

    KM1NDY Ham Member QRZ Page

    Hi:
    Thanks for your note! The only odd part about this antenna in regard to a 49:1 (which it very well may be) is that it is decidedly NOT multiband, at least in the since of a multiband endfed. It has a fairly sharp SWR dip at 20M and another lesser spike at 80M. This is in contrast to at least my homebrew 49:1 which has a much broader bandwidth of low(-ish) swr. Almost looks like a dipole on my analyzer. I will see if I can find some comparative pics.

    Just a note on this comment. I am still very surprised at the consistency. It is probably more my ignorance in a way with radio propagation, but the pattern really struck me.

    The orientation of the feedline is something I have only recently started paying attention to, as I am starting to really practically understand that the coax is often a part of the radiator. This antenna set up had a 90 degree off-set to the feedline. I may start playing around with chokes to see what that does to the antenna.


    Your feedback is greatly appreciated! Good food for thought!
    Best,
    Mindy
     
  5. KM1NDY

    KM1NDY Ham Member QRZ Page

    First off, thanks so much for this post! It is very helpful. There is so much here to take in, I need to return to this a bit later. These are my first thoughts as I make my way through, so incomplete. I am going to go through your model and talk later this evening.

    1. I commented to K7JOE on this, but in brief it may be a 49:1 unun, but the SWR analysis does not have a pattern conducive for multibands. It is largely a single 20M band antenna (with some resonance on 80M). This is what led me to think it may be a 9:1 instead, but I am admittedly mostly guessing here.

    2. I am definitely using the free version. My knowledge has not caught up to the pay version yet, haha!

    3.
    Very interesting. It actually then makes this antenna sound like a version of an off-center fed dipole. Possibly able to make (in essence) a "center-fed" dipole by placing a choke on the feedline to match the length of the radiator?

    4.
    I have to go through this entire section in more detail, and I plan on it. This is such an outstanding description of how to model this. Greatly appreciated!

    5.
    If this antenna behaves as a monoband on 20M, would that mean that at 14mhz the impedence could be lower than 2450 ohms? And therefore potentially not a 49:1 transformer? I am just not sure if this is the correct thinking in terms of impedence at different frequencies or not. Maybe don't expend time answering this, as I can tell the answer may be embedded in your response -- I just need time to process what you are saying!

    Thanks again for this incredibly descriptive post! I may have more comments later on.

    Mindy
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    A single band EFHW usually needs a 1:49 or 1:64 transformer.
    A multi-band usually needs a 1:49 and a matching coil. Read that part of the slide show linked to at the bottom of my QRZ page.

    An EFHW style of antenna is actually an "extremely off-center-fed" dipole, typically 5-20% of the dipole on one side of the feed-point and 80-95% of the dipole on the other side. In most cases, Common-Mode current on the feeding coax shield "acts" as the 5-20% part of the dipole, so there is no explicit antenna wire there.

    Actually, the theoretical feed-point impedance of a free-space (no-coax in the picture) 20m (14.2MHz) near-end-fed resonant dipole can be adjusted to any reasonable value. Say that we intend to eventually use a 1:49 Z-ratio (50:2450 Ohm ratio, 1:7 turns ratio) transformer. That implies that we would like to find a total dipole length Lft, and a position of the feed-point as a % of distance along the dipole P% where the feed-point Z is as close to 2450 + j0 as we can get it.

    A bit of work with the optimizer in AutoEz finds that a free-space dipole made of bare #14awg copper wire L=36.5ft with the feed-point P=10.35% of the way along the wire has a feed-point impedance of 2447- j1 Ohms.

    Suppose we wanted to use a 50:3200 Ohm transformer (1:8 turns ratio), instead. Then L=41.5ft, and P=19.9%. (Counter-intuitive, isn't it?)

    Suppose we wanted to use a 50:1800 Ohm transformer (1:6 turns ratio), instead. Then L=35.2ft, and P=8.5%. (Counter-intuitive, isn't it?)

    I uploaded a simplified version of a "near-end-fed dipole" ready to run in EzNec except that it has more than 20 wire segments so you will have to reduce the segment count to run it in the free version of EzNec.
     

    Attached Files:

  7. K1FQL

    K1FQL XML Subscriber QRZ Page

     
  8. K1FQL

    K1FQL XML Subscriber QRZ Page

    Hello Michael,

    Thank you for sharing your presentation and EZNEC files. I think that I am missing a key point, but when I iterate the transmission line length and the corresponding wire 4 length, the outputted results vary greatly. If the line choke is doing its job, should I not see a minimal effect when changing the transmission line length? I have not discounted the fact that the transmission line loss will pad the VSWR somewhat. I have also iterated the choking impedances that can be found online for products manufactured by Palomar, HyEndFed and MyAntennas. These vary greatly at 14 MHz. What am I missing?

    '73 and thanks,
    Marty
     
  9. W9XMT

    W9XMT Ham Member QRZ Page

    Below is a graphic showing what might be expected for the radiation pattern of the antenna system described in your blog, excluding any feedpoint matching losses that are present.

    [​IMG]
     
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  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Hi Marty @K1FQL,

    Which version of EzNec are you using (Free or Paidfor)?

    Zip the model you are having trouble with, post it here, and I will take a look at it.
     

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