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Loading coil wander lead

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB1CKT, Sep 14, 2017.

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  1. KB1CKT

    KB1CKT Ham Member QRZ Page

    On a lark I've been winding loading coils for a mobile HF antenna. I had seen the $20 ARRL article, figured, what the heck. I'll modify it for what I have. I had some finger stock (? not sure that is the right term, it's plastic wire spacer) and used a 1.25" PVC pipe for a 5.7" long coil with 0.25" turn spacing. The stock uses 0.125" but I skipped slots, as my Armstrong coil winding machine isn't very good. I got 21 turns, and... oddly enough, it hit 20m perfectly, installed into a 12m Ham-Tenna. With widely spaced turns it's probably not very good, but it was cheap enough.

    Last night I made another one, this one with 1/8th inch spacing and 40 turns, to see if I could hit 40m. However, I'm curious about the wander lead. All the plans I've seen have the lead from the low side of the coil, not on high side. I'm not sure it matters? Current is the same on either side of the coil, but the whip side is high voltage relative to low side--but the wander lead is all the same voltage.

    Attached Files:

  2. N5DMC

    N5DMC Ham Member QRZ Page

    Well, look at it this way, you could of over wound the number of turns and then then employ 2 tap leads. One on the top & one on the bottom. Good looking coils none the less :)
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Sorry, but that is an Old XYL's Tale promoted by the ARRL and others. The mobile antenna is a standing wave antenna. The forward current is approximately the same magnitude on either side of the coil and the reflected current is approximately the same magnitude on either side of the coil. But the total current is the phasor sum of the forward current and reflected current and the phases are quite different at each end of the coil. Here are a couple of graphs of the total standing wave current in a large loading coil. The first graph shows what The ARRL Antenna Book says vs what the helical coil modeled in EZNEC says. The second graph shows what the sum of the forward and reflected currents look like inside the loading coil when the phase is taken into account.



    Here's the background information.
  4. KB1CKT

    KB1CKT Ham Member QRZ Page

    Interesting. I have not taken the time to really think about this coil; my first though has always been, there can't be current at the top, as the whip is short and very capacitive reactance (small capacitance so little current to charge relative to the GND plane). But it's a coil with "good" linkage so it must act (mostly) like a proper lumped circuit component. Regardless, even with some radiation, a 20% variation is not that large; interesting that there can be higher current inside the coil than at either end! But lower current out of the coil meets my impression (which is not always the best to go by).

    I haven't read my handbook in a while, I'm kinda in the mood of "build it and they (DX) will come".
  5. KB1CKT

    KB1CKT Ham Member QRZ Page

    Thanks! I was kinda surprised at how easy it was. Next time I plan to tack the strips down with superglue, it was a pain chasing them around (and they tend to "wander" while winding). Household 14g wire, strips relatively easy with an Exacto knive pulled down it. Finding SS 3/8x24 hardware isn't the easiest; Home Depot didn't have any but I found some in a smaller hardware store. PVC is obviously easy to find. Only thing I need to find a source of is the 3/8x24 couplers; I'm reusing bits from older mounts. What I might do, when I figure out what I want, I might just find the turns count required, and wind a coil with that exact count. It's like an hour to make one of these coils.

    Ironically, MFJ sells a 4' Ham-Tenna mast, but it costs more than buying a regular Ham-Tenna 10m. Which would come with a 3' whip! I've toyed with getting one of their stubby HF antennas, just for the 2' mast, and then making a proper capacitance hat to go on top of that; but one step at a time.

    I guess that is the other source I don't have good access to: 3/8x24 fiberglass masts.
  6. KB1CKT

    KB1CKT Ham Member QRZ Page

    Finally got to test it. Online calculator says it is about 10.5uH. Seems to be at 9MHz with all turns used, 88ohm + j0, something like that. I do have an excessive amount of coax used between antenna and analyzer, 15' or so, but I think the coil is insufficient for 40m. A 2" form may be necessary, or more antenna mast.
  7. NH7RO

    NH7RO Ham Member QRZ Page

    Try auto parts stores for 3/8 x 24 ss bolts. Until Harvey demolished my local Discount Auto place they had plenty. Cut the head off and file any rough spots for making couplers.

    Wal-Mart, Ace and Home Depot carry the orange fiberglass driveway markers which are 3/8th inch O.D. too.
  8. KB1CKT

    KB1CKT Ham Member QRZ Page

    I think I can source bolts locally, but the couplers are harder to find. Female-female, used on top of sections. Amazon appears to be a source though, so I might just make up an order and get a number.

    3/8" diam markers, I'll look for those. I thought the driveway marker were 3/16" though, and not suitable for a loading coil at the top of them. Too much weight. Maybe I'm looking in the wrong place; next time I'm there I will look and see if they have fatter markers.
  9. KB1CKT

    KB1CKT Ham Member QRZ Page

    So now I have three coils wound up. One with a lazy 1/4" spacing for 20m, on a 1.25" form. One with 1/8" spacing for 30m, on a 1.25" form. And one for 40m, 1/8" spacing on a 2" form--that one, I got it wrong and it's got 2x the turns it needs. Need to make a second version as it's too heavy.

    Anyhow, inserted into a 12m Hamtenna, the 20m coil gives weird results. 2:1 SWR bandwidth of 740kHz:
    -13.71MHz is 48ohm +/-j30
    -14.00MHz is 50ohm+j0
    -14.45MHz is 25+/-j1

    Lazy winding? I'll blame the setup, as it's not ideal (trunklid, which is bonded to car body, using shunt 9T inductor at base of antenna). Too much GND loss I think, being on the trunklid. [It is not using the magmount for capacitive coupling.]

    The 40m coil into same 12m Hamtenna gives 83kHz of BW:
    -7.023MHz is 26ohm+/-j10
    -7.059MHz is 39+j0
    -7.106MHz is 79ohm+/-j29

    [MFJ doesn't give me sign of reactance.]

    Attached Files:

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