The ARRL End Fed Half Wave Antenna Kit and doing it cheaper and better

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Jun 28, 2021.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: MessiPaoloni-1
ad: HRDLLC-2
ad: abrind-2
ad: L-Geochron
ad: Left-3
ad: Left-2
  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Mike Mladejovsky, WA7ARK June 28, 2021

    The ARRL is offering an End Fed Half Wave 40m, 20m, 15m and 10m antenna kit through its on-line sales department. (see http://www.arrl.org/end-fed-half-wave-antenna-kit ). ARRL has partnered with a Dutch antenna kit maker (see https://www.hfkits.com/build-instruction-impedance-transformer-250w-for-end-fed-antennas/) and are re-selling that kit. The ARRL touted this kit in the 2021 ARRL Field Day Guide which was included as a supplement in the June 2021 QST, which has full page ad for the kit on page 4 of the supplement.

    I bought the kit, built it, improved it, tested it, and have some guidance for folks that would like to use it on the four bands it is claimed to work on. The ARRL just refers the kit buyer to the HF-kits website. I found the instructions on how to wire the transformer to be OK, but HF-kits instructions on how to make the antenna work on four bands is confusing, disorganized, and it is obvious that they do not understand their own product! Shame on the ARRL for promoting the product without adding value by supplementing the vendor's meager instructions...

    The quality of the parts is very high. It is basically 70ft of insulated wire for the radiator, a plastic box, a FT240 type 43 FairRite toroid, enemaled wire for winding on the ferrite toroid, a 100pF capacitor, and hardware. Since an EFHW antenna like this is so simple, I would normally just build one from parts, but I specifically bought the kit just to do this write-up. Later in this article, I will have some comments about home-brewing one without the kit. Here is what the HF-kits transformer looks like after I built it:
    HF-kits.png

    Assuming you have built the transformer as instructed (or using my "improvements"), here is how to turn the kit into a useful, 4-band antenna: For testing, I temporarily suspended the antenna horizontally between two 38ft masts. The transformer and radiating wire is held aloft by tension in two cords. I fed it though a 100ft run of RG-8x coax, where the coax hangs down from transformer for about 33ft, and then lays horizontally for 67ft on the ground as it runs to my test set-up. Here is a picture looking toward the antenna:
    Set-upAnno.png
    For the initial test, I just took the full length of the supplied wire (68.5ft) and suspended it as shown. Using the HF-kits transformer and 100ft of RG-8x, the un-cut wire resonates at 6.732MHz. I am measuring with my RigExpert AA-600. The Z=58.5+ j5.8 Ohms, for a Swr50=1.21 (Return loss > 20 dB). Obviously, the un-cut wire is too long.

    Without regard to 20, 15, or 10m, I next scaled the wire length to move the resonance up into the 40m band, using this calculation: NewLength = 68.5ft * 6.732MHz/7.15MHz = 64.52ft, which requires removing ~4ft of the wire... That moved the resonance into the 40m band. Next, I found the three harmonic frequencies where SWr50 is minimum. Here is the tabulated data:

    UnCompFres.png

    Obviously, even though the fundamental resonance lands in the 40m band, and the harmonics have very low Swrs, the harmonic frequencies land far outside their respective bands... (the actual measured wire length was 64'4", after accounting for ~3in of lead length inside the transformer box). Uncompensated, the antenna is usable only on the 40m band.

    HF-kits suggests using a coil to align the harmonics, but they are vague about how long the wire, how much inductance, and where the coil should be placed. I have previously studied harmonic alignment on EFHW antennas (I call it "compensation") and immediately concluded that I know how to do it better! An optimally placed series-capacitor will do the trick.

    Time to use EzNec and AutoEz. First thing I did is create a model which faithfully reflects the insulated wire, the transformer, the 100ft of RG-8x (which simultaneously transforms the impedance, slightly reduces the System SWR due to coax loss), and most importantly, accounts for how Common-Mode current on the outside of the coax acts as the required counterpoise for the antenna.

    I tested my EzNec model first by modeling the 64' 4" wire described above. Got similar results to the actual antenna, especially the uncompensated harmonic frequency alignments. Next, I tasked the AutoEz Optimizer to find a wire length, capacitor position along the wire, and capacitor value that minimizes Swr at 7.15, 14.175, 21.225 and 28.4MHz simultaneously.

    The modeling and optimization results showed that a 195 pF capacitor placed 26ft from the transformer on a 68' 1" wire would better align the three harmonics while preserving the fundamental response in the 40m band. Note that this model provides three variables for the optimizer to work with.

    I had a 200pF 500V Silver Mica capacitor in my parts stock, so I made a compensation network consisting of the 200pF capacitor in parallel with a 33K 1/2W carbon resistor, cut the wire and inserted the network. The network looks like this:
    CompNetwork.png
    This was inserted inside a "hood" made of 1/2" PVC water pipe cut to a length of 1.75":
    CompHood.png
    The antenna wire can now be looped and tied off through the two holes, and the yellow wires soldered to the two wires. This keeps the sun/weather off the network, and the hood acts as an insulator capable of supporting the wire axial pull.
    WiredHood.png
    The resistor in the compensation network shunts the capacitor to equalize the electrostatic charge between the isolated wire section and the wire section that is grounded through the coax/transformer. This prevents static-induced breakdown of the capacitor dielectric.

    The modeler calculates that the highest voltage across the network (with 100W of RF applied to the antenna) occurs on 40m and is ~85Vrms, where the maximum RF current through it is <1.2Arms, so the SM capacitor is well within its ratings. It might have to be replaced with a doorknob cap at higher power levels.

    It took two iterations of shortening the main wire 3inches each time to produce the following Swr50 minimums with the antenna suspended at 35ft agl as shown in the first image: This is as predicted by the EzNec model.

    ...continued in part two:
     
    WA3YMH, WM3O, 2E0CIT and 4 others like this.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    ... continued from post #1...

    M40.png

    M20.png

    M15.png

    M10.png

    The final wire dimensions and component values, after trimming the overall wire length to get the Swr plots above, has a 200pF 5% 500V Silver Mica capacitor, shunted by a 33K 1/2W carbon resistor. The RC network is located 25ft 10.5in from the transformer. The total wire length from transformer to insulator is 67ft 3.5in. This shows an eminently usable multiband antenna that will not require a tuner.

    To make sure that the unmarked toroid supplied in the kit was indeed a FT240 size type 43 material, I build a second home-brew transformer with a core that I had on hand. That came from: https://www.kf7p.com/KF7P/Ferrite_chokes.html (see the "type 43, 2.4in O.D. for $6.20).

    I built it on a piece of white Vinyl cut from a section of rail for a vinyl fence. Looks like this:
    arkTrans.png
    There are some differences in how I build my 1:49 transformer compared to the HF-kits version. I did not have a 100pF capacitor on hand, so I used two 200pF SilverMica caps in-series (same type of capacitor as was used in the frequency compensation network).
    Notice the absence of parallel, twisted wires for the two primary turns. I used 2 turns of 16awg, silver-plated TFE insulated wire. The secondary winding consists of 12 turns of #18awg enamled copper wire (the cross-over counts as a turn). It is wired in series with the 2 turn primary, effectively making a 2:14 turn (50:2450 Ohm) auto-transformer.

    The use of twisted primary turns utilized in the HF-kits is all over the internet. Nobody ever has justified that there is any engineering benefit to building an auto-transformer that way! The reality is that to reduce losses, the two primary turns should be heavier wire gauge because the current in those turns is seven times higher than the current in the 12 secondary turns, meaning they can be wound with smaller wire. The use of the cross-over turn shows how it puts the output of the transformer diametrically opposite the primary, keeping the wires as short as possible, and stray capacitance to a minimum.

    My version of the transformer can be homebrewed for a parts cost of ~$10. I repeated the swr tests using the same compensated radiating as used in the earlier tests, and the SWR minima are within 10kHz of what was measured using the HF-kits transformer.
    Notice that my ferrite coe is not enclosed inside a hermetic box; rather I suggest leaving it open so as not to trap heat in a sealed box, and just spraying the assembly with a clear conformal coating, like Krylon. I will deal with core heating at high RF power levels and/or high duty cycles in a future article.

    As a separate experiment, I wound a single-FT240-43-core 1:49 transformer with a three-turn primary and a 18 turn secondary and tried it with the compensated wire. The Swr50 on 40m was comparable to the 2 turn primary version, however, the Swr50 on 20m, 15m and 10m is much higher, over 2.5 on 10m... My conclusion is that I would be inclined to use the 3 turn primary on a multiband antenna slated for 160, 80, 60 or 40m, but no higher than 20m. The two turn primary is optimum for a 40-10m antenna.

    In the past, I have advocated putting a coax Common-Mode choke on the coax a few feet from the transformer to control CM current on the coax, and to separate the radiating part of the antenna from the feedline that you do not want to radiate. This is necessary depending how long the coax is, and what sort of path to earth ground exists at the station end.

    I was prepared to add such a CM choke to this set of experiments. I even build a CM choke at the same time I built my version of the 1:49 transformer. Here is the one I built using a FT240 type 31 ferrite core. The white coax is a bit smaller diameter than RG-58, is 50 Ohms, and has Teflon outer insulation and a Teflon center insulator, and has Silver-plated conductors. I have a large coil of this, so I have been using it to make my own chokes/baluns. This one can be inserted in-series with coax, or could be used as the center insulator for a conventional dipole antenna (the wires would connect to the two machine screws)
    Choke.png

    I have a homebrew instrument to measure Common-Mode current on coax that I built and calibrated several years ago. It is designed to slide over a PL259, so can be used on any RG-8 or smaller coax cable. The readout is a 50uA DC meter. It is mounted on an insulating handle, used to slide it along the coax to find where the CM standing wave peaks.
    CMmetercoax.png

    I tested the compensated EFHW antenna at 100W using a FT-857D transceiver powered from a car starting battery. I did this specifically so that the transmitter/battery is completely isolated from AC power, and the AC power ground system. This meant that I could add a short bonding wire which alternatively makes or breaks a ground connection to my huge metal building to see what effect that has on the CM current that flows on the coax or bonding wire near the transceiver (and indirectly on antenna SWR and/or impedance).

    First, using 100ft of coax to feed the antenna, I was surprised how little CM current exists on the coax right near the transceiver, even when the transceiver chassis is bonded to the building. The 100ft coax length I used happens to avoid a "CM standing-wave resonance coax length", so it makes little difference if the bonding wire from transceiver to building is connected or not.
    There was less CM than I expected, so I was curious why?... About 65ft of the RG-8x coax was laying on the cement apron in front of the hangar, which undoubtedly is reinforced with steel rebar. I moved that 65ft of coax so that it lay on bare dirt instead of concrete, and that increased the CM current through the bonding wire into the grounded building. I concluded that laying 65ft of coax on the concrete was acting as a common-mode choke (as is burying coax).

    The magnitude of the CM current flowing into the building with the coax on dirt is still so small as to be inconsequential for a practical antenna installation, and that is evidenced by only a slight change in SWR with the bonding wire open or closed, so for this particular coax/antenna configuration, the antenna does not need a CM choke...

    This is supported by the fact that thousands of these antennas have been deployed, yet very few of those installs end up having to add chokes to control CM after the fact. This is because those problems greatly depend on accidently having a critical (resonant) coax length. The probability of making the coax the critical (wrong) length is small.I will also deal with CM issues in part two of this paper...

    So, to summarize this part of the work, I have shown the compensation network that is required to make the ARRL/HF-kit into a useful 4 band antenna. I further show, that if you are capable, you don't need the kit, just home-brew the entire antenna from scratch. I also suggest a better ways to fabricate the transformer, and show a way of leaving the transformer open to eliminate trapping heat inside a box... More to follow.
     
    Last edited: Jun 28, 2021
    KJ7RDV, WA3YMH, WL7PM and 19 others like this.
  3. SWL37632

    SWL37632 QRZ Member

    Excellent work and report write-up.

    Your compensation method to sync-in the other harmonic resonances was very educational.

    Thank you for sharing.

    Question: I presume that your CM detector is a classic single diode envelope detector ? Otherwise, kindly instruct.
     
    KX4OM likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    SWL37632 likes this.
  5. K1VW

    K1VW Premium Subscriber QRZ Page

    Ver well done sir!

    Do you think using an FT240-31 instead of group 43 for the transformer toroid would substantially change things? That’s what I happen to have on hand.
     
  6. G0KDT

    G0KDT Ham Member QRZ Page

    I simply don't know how you manage to get these EFHWs to work.

    I moved from a Marconi longwire with 9:1 transformer to making a 49:1 and an EFHW. The only difference I have is that I have a 110uH coil and an extra c.2.2m of wire to give access to 80m, remove that addon and its pretty much the same antenna.

    They receive ok but the acid test is the signal reports and simply playing with WSJT-X ft8 its plain to see the reports I am giving others is way higher than I ever get back. Most of the time operation is rather like Tx'ing at 100w into a blackhole and nothing coming back.
     
  7. K9RJ

    K9RJ Ham Member QRZ Page

    Great article Mike. Super helpful for those of us without antenna modeling expertise.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I just noticed that it is not clear that the four SWR plots in post #2 are not model results; those plots are actual measurements made with a Rig Expert AA-600 Analyzer looking into the rig-end of the RG-8x. The test antenna, transformer, and coax are suspended between the two 38ft poles.
     
    Last edited: Jun 29, 2021
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    I have never tried to build a transformer using type 31 material. AFAIK, type 31 is too lossy to be used for transformers. It is specifically formulated to make chokes.
     
    AK5B likes this.
  10. KC3GHK

    KC3GHK XML Subscriber QRZ Page

    Wow, great write up, thanks Mike. I have printed it out for my future use in building my own HWEF and for all the beginner useable information on matching, and building transformers. Thanks again Mike for sharing all your knowledge and making it understandable for us non electronic folks! I have learned so much through your posts. AndyT
     
    2E0CIT likes this.

Share This Page