160meter EndFed HalfWave Antenna on a 65ft tower

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Jun 13, 2019.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    My buddies at my former ham club have access to one of the obsolete ATT Long-Lines cross-country Microwave sites, now owned by a private party. Looks like this:

    [​IMG]

    They have installed an internet-remote-controlled HF radio at the site, primarily to allow retired club members living in HOAs to have access to HF. The original installation had only a G5RV type of HF antenna. They wanted to add 160m to the second antenna port of the radio, and approached me to come up with an antenna.

    Drawing on the experience I gained while experimenting with EndFed antennas for a hamfest presentation, I figured that if I built a resonant 160m EndFed, then it would also provide coverage of 80m, 60m, 40m and possibly some higher bands. Because of local terrain, a wire antenna fed from the tower end is preferred.

    The tower is ~65ft high. There is not another high support, so the feed-point of the EF would be at the tower top, while the other end would be fairly close to the earth on a low support. There is a gully close to the big tower. Beyond the gully is a ridge, so the wire would hang mostly over the gully, but it would slope down from the tower top.

    I had on hand a MyAntennas mef-110-3k 1:49 transformer, so I based the antenna on that. I ordered 270ft of #14AWG from the Wireman. I first built and tested the antenna at home, elevating the driven end up my 55ft tower, letting the other end slope downward.

    First attempt, I mounted the transformer 53ft up my TX455 tower, connecting the "Gnd" wingnut on the transformer to the grounded tower leg, with coax routed down the tower leg to the shack below. This effectively makes the entire metal tower (TX455) the "counterpoise" for the EFHW.

    Didn't work very well: The lowest SWR was about 3.7, and the resonant point was down at 1780kHz (hadn't pruned the wire yet). Correcting for the coax, the feedpoint impedance at lowest SWR was 15.8 - j17.5, which means that there is way too much "counterpoise" (the entire grounded tower and everything on it).

    Recalling that simulations show that there need only be a tiny counterpoise (<0.025wl) to make the feedpoint impedance ~2500 Ohms. If the impedance connected to the transformer secondary is on that order, the impedance at the primary of the transformer is ~50 Ohms, meaning that the SWR on the coax would be low. I set about to make the counterpoise much smaller by effectively isolating the antenna from the tower.

    Here is the justification for the "too much counterpoise" statement: This is from a simple EzNec simulation of a half-wave wire fed ~6% from the end, where I optimized the lengths to get a SWR50 at the transformer primary as close to 1 as possible.

    NearEFstub.png
    Note that to make the radiating element one half-wavelength long, it takes only a tiny "counterpoise", i.e. the 8.5ft wire stub to the left of the feed-point. This illustration is for 80m, so double the dimensions shown above to re-scale the illustration for 160m. The current distribution (pink line) is identical to what would happen on a center-fed half-wave dipole. Obviously, removing the stub and replacing it with a massive 65ft tower is not the thing to do. So much for all the bogus advice proffered here about End-Fed antennas needing massive "counterpoises"!

    I tried suspending the transformer on a rope, offsetting it from the tower, and then relying on 14ft of coax from the transformer to tower to act as a short counterpoise. Initially, I had the coax shield bonded to the tower, but that didn't do anything to reduce the SWR, because the tower is still part of the circuit. It occurred to me to put a common-mode choke on the coax just before it is bonded to the tower, making the total length of the horizontal antenna equal to about 14ft of coax shield plus 270ft of wire beyond the transformer. This effectively "disconnects" the horizontal wire from the vertical grounded tower at RF, but leaves it grounded to the tower at DC for static drain.

    I made the choke by winding about 22turns of RG400 (the amber-colored teflon coax) through a 240-31 core, which measured about 125uH using my AADE Inductance meter. Putting that in-series with the coax near the tower brought the SWR50 down to 1.39 @ 1797kHz. I tried different lengths of coax between the choke and transformer, varying from 8.5ft to 26ft, and 14ft is where the SWR is minimum. This leaves pruning the antenna to a better frequency inside the band, and checking the alignment of the harmonics with the higher frequency bands.

    Here are the SWR50 nulls at the shack end of 52ft of RG8 coax with the CMC, 14ft of coax to the left of the transformer, and 270ft of wire to the right:
    Code:
    freq   R   jX   SWR
    1797 45.7 -15   1.39
    3516 48.6  4.3  1.1
    5335 49.8  0.3  1.01
    7218 39.7 -14.1 1.47
    
    Obviously, if I shorten the wire, I can bring the fundamental resonance up to ~1860kHz, but then the 4th harmonic would land above the 40m band. I used an EzNec simulation and AutoEz optimization to come up with the final overall wire length and a compensation coil inductance and placement that makes the antenna more use-able.

    Here is a sketch of what I ended up with:
    mYEFHW.png

    The optimized antenna was them installed at the old ATT site. Here is a video taken during the work party.

    https://photos.google.com/share/AF1...?key=NmZ0cTNXRUMwSGEwVU1uWlBtbmc0aHUyQVF5dHdB

    My friend Clint made the diplexers to allow sharing an existing 1" heliax feedline running up the tower. Here is a write up about how he made the two diplexers:

    https://ka7oei.blogspot.com/2019/06/using-same-feedline-for-hf6-meters-and.html

    The final installation allows the tuner built-in into the Kenwood radio to tune any frequency between 1800 to 1900, all of 80m and 30m, while the tuner can be switched out of circuit on 60m, 40m and 20m. The Swr did not change much between my test location and the final installation.

    Since the existing G5RV and new EFHW antennas overlap on most bands, and they are installed at about 45degress apart in azimuth, it makes it possible to select the one that provides the best coverage in any given direction.
     
    Last edited: Jun 13, 2019
    W0PV, N0YPD and K6BRN like this.
  2. AA5CT

    AA5CT Ham Member QRZ Page

    Measured capacitance primary to secondary (unless the primary winding's ground is connected to the secondary winding's ground) on that 49:1 transformer (stray capacitance is something NEC computational engines don't know about)?
     
    W0ZS likes this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    ...but can be added to the transformer model, if you know what they are...

    The leakage inductance from primary to secondary can also be modeled, if you have a network analyzer to characterize the transformer. I just bought a DG8SAQ-3E.
     
  4. AA5CT

    AA5CT Ham Member QRZ Page

    Full S-parameter capability is kinda overkill on HF where lumped component values suffice; this is where an old General Radio bridge comes in handy too.

    At HF one can also use a dual-trace O-scope to secure S21 values, and a simple resistive bridge supplies S11 values.

    People shouldn't be scared off doing this kind of work, rather simple devices can be used to perform these measurements, and once any given measurement is made the 'data' representation type can be converted into any other form for use in computer-based sim or modelling tools in the form they require.
     
  5. AI3V

    AI3V Ham Member QRZ Page

    So a inductor on one side of your antenna is a "loading coil" and on the other side its a "choke"?

    Wow, just wow.

    Rege
     
  6. AI3V

    AI3V Ham Member QRZ Page

    Its much easier to model a antenna when you guess what the parts do.....

    Rege
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    No Rege, it is much easier to claim you know everything about antennas than it is to show how they work by modeling them first, and then actually building and testing them. Which do you do?
     
    KK4NSF likes this.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    You obviously miss the irony! The ferrite common mode choke is there to disconnect the antenna from the "counterpoise" you keep insisting that this antenna must have to work.

    The coil is a simple addition to move the resonances into the center of several higher harmonic bands. Do you need me to explain how it works, Rege?
     
    KK4NSF likes this.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    I got a question from Glenn @W9IQ about putting a choke on the coax feed-line about 0.07 wavelength down the coax of an EFHW antenna, such as I used in the 160m EFHW I built and described here in post #1. I subsequently noticed that the European maker HyEndFed recommends putting a Choke 0.5wl from their 1:7 transformer when installing their antennas.
    ...
    Putting a Common Mode Choke=CMC about 0.07wl down the coax from the transformer does put a high impedance there at RF frequencies, forcing the current to be nearly zero through the CMC, creating a voltage drop of up to thousands of Volts across the CMC, but does not provide galvanic isolation. The antenna can still be effectively grounded for static drain through the coax provided that the coax shield is dc grounded somewhere...

    Actually, the antenna current distribution along the antenna wire has the same symmetric Sinosoidal distribution as would exist on a dipole. It is the asymmetric placement of the coax feedline with respect to the antenna's center that induces current on the outside of the coax.

    Placing the CMC as described in post#1 effectively breaks the antenna system into two isolated conductors, call one the "antenna", the other the "feedline".

    Just as with a Hertz dipole or a Marconi vertical, the goal is to maximize current flow along the antenna conductor, while simultaneously minimizing the current flow along the outside of the feed line.

    This is a matter of degree. It is only when the peak current in the feedline becomes more than about 20% of the peak antenna current that bad things happen...

    To minimize the CMcurrent on the feedline, you have to understand why a common-mode-current standing-wave forms along the feedline, beyond the spot where it was forced to zero by placing the CMC.

    As Glen rightly points out, it is "mutual inductive coupling" between the antenna and the feedline that creates the common-mode-current standing-wave. This is the same mutual coupling that makes Yagi antennas possible, where one element is directly driven, but the parasitic element has a current standing wave in it due just to mutual inductance between the elements.

    In the Yagi context, the parasitic element is parallel to the driven element, while in coax-fed, end-fed horizontal wire antennas, the coax is usually at right angles to the driven element. It is harder to induce a current into the feedline if it runs at right angles to the wire, although in the antenna in post#1, it runs in-line with the antenna.

    Only a narrow set of conditions will induce sufficient current into a parasitic element, so that the feedline acts as a parasitic element in the Yagi sense. Its length has to make it nearly resonant!

    By "nearly", I mean that if you were trying to actually build a two element Yagi, the parasitic element would have to be within +-3% of the resonant length. (- means it becomes a "director" and + means it becomes a "reflector").

    So, if you put a random length of coax with random routing between the CMC on the End-fed antenna and the rig, what is the probability that it is resonant?

    To be resonant (so that it will act as a parasitic element in the Yagi sense) you have to know what is happening at the rig end of the feedline. Two possibilities: the rig is completely floating with respect to RF ground (unlikely) or that the rig is either explicitly grounded (ground rod at wall entrance) or is implicitly grounded to the electrical service through a third prong on an AC cord (likely).

    The floating case might occur with SOTA-type battery-powered operations; let us talk about the AC powered station case:

    Any length of coax, grounded near the rig end (either a bit up-steam or down-stream of the rig) has the properties of a Marconi mono-pole antenna. Assume that at least part of it is vertical, the rig end is grounded, the other end is open (because the CMC is placed there).

    If, and only if, the coax mono-pole length is within a couple of percent of being resonant will it develop a common-mode-current standing-wave of sufficient magnitude to substantially effect the pattern of the antenna. To hit resonance, you have to accidentally make the coax an odd multiple of 1/4 wavelength long.

    For the record, it is possible to build a coax-fed, center-fed horizontal dipole where the feedline turns it into a Yagi! All you have to do is connect the coax shield directly to (say) the right leg of the dipole (i.e. do the DS cheap-bastard thing and leave off the CMC at the feedpoint), and make the rig-end grounded coax an odd multiple of 1/4 wavelength long. This will unbalance the dipole such that the current in the right leg of the dipole might be 20% (and the Common-mode current on the coax shield would be 80%) of the peak current in the left wire.

    In this regard, dropping the coax down below the transformer end of an EFHW antenna is only slightly worse than the center-fed scenario in the previous paragraph.

    The mitigation is the same; add a ground rod, change the length of the coax, add a second CMC, etc.

    I have demonstrated this both by using simulation, and doing an experiment where I purposely changed the length of the RG8 coax feeding an EFHW-8010 commercial antenna that I was testing.
     
  10. W1GHD

    W1GHD XML Subscriber QRZ Page

    Whenever I see one of those old AT&T towers, I have to wonder if I handled the waveguide. I did electrical and mechanical inspection at a company that made waveguide for Western Electric, better than 1,000 pieces per month.
     

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