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Icom AH-4 + 9:1 unun with efhw?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VE7TBN, May 25, 2021.

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  1. VE7TBN

    VE7TBN Ham Member QRZ Page

    Hi folks.

    I searched via title but didn't find what I'm looking for. I already have an AH-4, Icom 746 Pro and LDG 9:1 unun so.....

    Will a 9:1 unun step down a 40 meter EFHW horizontal wire impedance to a range the AH-4 can handle? Assuming open wire or ladder line would connect unun to AH-4, would using 5' - 10' of that type of feed line raise impedance beyond what the AH-4 can handle? i.e. especially if one made their own open wire feed line?

    Goal: "portable" type NVIS rig in a small camper van. With the AH-4 in roof top luggage rack, horizontal antenna wire could be anywhere from 6 ' - 18' from ground. I could also do a sloper type configuration with AH-4 on or near the ground.

    Though not ideal, I can make an air wound choke with the 50 Ohm coax feed line if that would help this antenna system.

    I have the SGC SG-230 manual. My understanding is that some ideas in that manual are quite relevant to the AH-4.

    Apologies in advance if this topic has already been discussed.

    73,

    Neil.

    LDG unun

    [​IMG]
     
  2. W9WQA

    W9WQA Ham Member QRZ Page

    my stock answer to,
    "will this work?" is.
    try it and let us know...
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    An EFHW for 40m is 65' long.

    Is that what you have? Is it 65' long, or is it just an end-fed that's shorter?
     
  4. N8AFT

    N8AFT Ham Member QRZ Page

    Everyone I've knon to use a single piece of wire and the AH4 never used an un-un.
    Please refer to the AH4 manual for the correct answer...
     
    KG7WGX, WG8Z and K0UO like this.
  5. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    Yep.

    The AH4 manual has a section on undesirable antenna lengths and does not show the use of an unun.

    Why not just cut the antenna to a workable length and ignore the unun?
     
    WE4E likes this.
  6. WA7F

    WA7F Premium Subscriber QRZ Page

    Probably not a good idea. I mean you can make anything work but, I suggest using your AH-4 for a “random” wire antenna. It could be a horizontal, vertical, Inv-L, sloper. Basically, whatever fits the situation.

    Check this out:
    https://udel.edu/~mm/ham/randomWire/

    Good choice on the AH-4 for portable OPs up to 100W. For POTA, car camping or RV stuff it’s a good fit.
     
    KC9UR and N8AFT like this.
  7. N8AFT

    N8AFT Ham Member QRZ Page

    If it's not broken then you don't need to pay money to "fix" it!
    The folks who design this stufff know more than you may think...
     
  8. KI8DJ

    KI8DJ Ham Member QRZ Page

    Ditch the unun completely unnecessary. Use a non resonant length of wire as long as you have room for. Provide some radials. Or a simple counterpoise to the ground lug of the tuner.
     
  9. VE7TBN

    VE7TBN Ham Member QRZ Page

    Thanks everyone. The consensus seems to be that the AH-4 + unun etc. would be less efficient, or not work properly at all, vs using the AH-4 as designed.

    [edit] I use a home made 1/4 GP vertical and have had QSO's on 20 meters with operators using EFHW antennas (some QRP). But, a recent clear and relatively long QSO to a "100 Watt" operator in Idaho (~ 620 miles from me) using a 6' high horizontal EFHW prompted me to revisit making a mono band EFHW to use with my VW camper "shack".

    I thought using parts I had on hand might make a better performing EFHW vs the AH-4 + various shorter "good" random lengths I've tried out; they did not work well at all on 40 or 80 meters. Band conditions, possibly poor counterpoise designs aside, a longer wire higher from ground certainly would've helped.

    The AH-4 manuals' statement saying not to use it with a 1/2 wave wire caused me to wonder if the main issue with doing so was that the impedance of the wire would be too high for the AH-4 to deal with and that a 9:1 unun would help negate that.

    I'll find a way to deploy a longer wire.



    73,


    Neil.
     
  10. WA7F

    WA7F Premium Subscriber QRZ Page

    Neil,
    I think the intent of not using the AH-4 with a half wave length of wire is to avoid a voltage feed. The EFHW that are so common right now are using matching transformers of 49:1, 65:1, etc to cope with the voltage feed that is present at the end of the half wavelength.

    The AH-4 is not designed to cope with a voltage feed but somewhere between a voltage fed dipole and a current fed. It will provide about a 10:1 match. If you look at the UDEL link that I posted it will provide you with wire lengths that work well with tuners like the AH-4. The goal of a “random” wire antenna is really to avoid half wavelengths or resonant lengths on as many bands as possible to provide a wide range of frequency agility.

    So “good” wire lengths for as many bands as possible, considering that you have have bare wire or you are adjusting following lengths by the velocity factor of the wire are approximately:
    67 to 74’
    104 to 109’
    134 to 137’
    170 to 173’
    201 to 205’

    I personally like 130 to 131’ of insulated wire in an Inv-L configuration. The ~130’ length puts the VF adjusted length or “bare wire” length into the 134’ to 137’ range. Keep in mind that the “random” antenna will perform best on bands where it’s near 1/2 wavelength or longer. A 130 to 131’ hunk of wire and a few radials of whatever number length you can do will get you on 160m to 10m with best performance on 80m up through 20m. You could still do very well with a 70’ length of wire, just try for as much height as possible for each situation.

    If you are operating mobile from an RV, each site will be a little different. Maybe just a sloper or Inv-V at one site and another with huge trees might allow for a vertical or Inv-L. If I ever retire I might make a habit of this. Just don’t forget to pack a decent launcher and some extra wire for at least four radials.
     
    W9IQ likes this.

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