Portable Doublet-Building Party & my 2022 Activation Challenge!

This past weekend, my good friend Vlado (N3CZ) and his wife came over to the QTH for the afternoon. It’s been a while since our families got together, so it was fantastic to hang with them.

I’ve been trying to tempt Vlado to do more field activations–we’ve done a number together in the past and it has always been loads of fun. Thing is, both of us have pretty active family/work lives, so it’s challenging to make schedules work out.

Nonetheless, our 2022 goal is to do at least one activation per month as a team!

MK-911

If you recall, a couple months ago, I posted an activation report and video using my buddy Eric’s 40-10 meter doublet. I called it a “stolen” antenna because it had been on loan to me for so long, I think Eric forgot it even existed.

Exhibit A: The stolen antenna.

Eric’s doublet was build around a Hughes Aircraft MK-911 Dipole Fixture that was designed and manufactured for the US Military and appears to have been part of the PRC-74 manpack radio-set.

I had assumed Eric found this as a one-off at a military surplus sale. Turns out Eric (and a few readers) made me aware that it was available at Fair Radio Sales in Ohio for $10.

Image of the MK-911 from Fair Radio’s product catalog.

In fact, here’s a link to the MK-911.

I decided to buy two of them: one for me, and one for Vlado. The temptation was strong to purchase a few more just for the 30 feet of 72-ohm military-surplus twinlead, however I understand that there’s a limited inventory and wanted others to be able to purchase this gem.

I was also thinking this antenna fixture would pair beautifully with Vlado’s Yugoslavian RUP-15/PD-8 manpack or even his IC-703 Plus.

Doublet time!

While our wives were catching up, Vlado and I made our way to the storage shed and opened my antenna parts boxes.

I did a little digging and found what I was looking for: some wire I purchased at a thrift store many years ago.

This wire has a black jacket that’s quite slick. Guessing it might be 20 gauge and might even be teflon coated. It was ideal for antennas and eyeing it, I thought there might be enough for two doublets.

For a Norcal-inspired doublet–which covers 40-10–we would need two 22′ legs. I decided (prior to cutting) that I wanted our doublets to go as low as 60 meters (5,332 kHz) and cover everything above. 60 meters is such a useful band. Thing is, I hadn’t done research into suggested leg lengths in advance.

We decided to pick a longer non-resonant length and just give it a go. If it worked, great–if not, we’d cut them down to 22 feet and be happy with 40M and up.

We cut the legs to 31 feet, so there’d be a total of 62 feet of wire in each doublet. Many thanks to my daughter Geneva (K4TLI) for helping stretch, measure, and cut the antenna wire with us!

Assembling the antennas was incredibly simple as there are built-in binding posts attached to the twin lead on the winding fixture.

Vlado and I both decided to use the winder as the center-insulator of the antenna. This is actually how this military fixture was designed to be used. The negative, of course, is that the center insulator is relatively heavy. This isn’t a problem for me at all since I use super strong arborists throw lines to deploy my antennas.

From WD8RIF’s website.

Eric (WD8RIF), by the way,Ā  actually detached the twin lead from the fixture and posts on his unit and built a new center-insulator from a discarded 35mm film canister (see photo above). He wanted to keep the weight down so he could support the center of the doublet on his fiberglass masts.

Testing QRV!

I had planned to hook up the doublets to my RigExpert antenna analyzer, then I realized it was essentially an unnecessary step.

The big question for me was, “Will my Elecraft KX2 find impedance matches on 60M and above?”

Vlado and I connected the doublet to the KX2 and tuned to 5,332 kHz. After confirming the frequency was clear, I pressed the ATU button. The KX2’s internal ATU churned for a couple of seconds and confirmed a 1.4:1 match.

Score!

We checked all of the bands above 60M and the matches were even better.

Standing in the middle of my driveway, I asked Vlado to load the POTA.app website and look for CW spots.

I couldn’t put the antenna away without chasing some parks!

We then proceeded to work about three stations on the air in CW with 5 watts. All of them gave us 599 reports!

Vlado chasing a POTA activator.

It was serious fun.

As I mentioned to Vlado, it might have been the first time I’ve ever used an HF “Handy Talky” with a doublet antenna!

In the end, we both walked away with two effective military-grade field doublets. A perfect antenna for our monthly “Team Baklava” activations.

2022 Activation Challenge

Last year, my personal challenge was to validate all of my park and summit activations with 5 watts or less.

Since I’m very much a QRPer and primarily a CW op these days, this turned out to be low-hanging fruit; lower than I would have guessed in this part of the solar cycle.

For 2022, I plan to continue the 2021 five watt challenge and add another layer…

This year, my challenge will be to build a new antenna each month and deploy it at least once during that month during a field activation.Ā 

The MK-911 doublet will count as January’s antenna.

I’m going to allow myself to build these antennas from anything and everything. I might even cannibalize a few of my broken/worn-out antennas.

I do want to keep these projects quite simple and easy for anyone to build in one sitting. I’ve gotten so much positive feedback from the super-simple speaker wire antenna I built in the field last year no doubt because it was so easy to build and super cheap.

I already have enough ideas to take me through the summer months, but I plan on dusting off some of my antenna books for more ideas!

With that in mind, I welcome your input! If you have a suggestion, feel free to comment!

17 thoughts on “Portable Doublet-Building Party & my 2022 Activation Challenge!”

  1. The Antenna-a-Month is a nice addition to your 2022 Activation Challenge, Thomas. I like it !
    As far as the doublet antenna, do you have any thoughts or guidance about how other forms of “twinlead,” such as 450 ohm ladder line, 300 ohm twinlead, lamp cord, or even two conductor phone line cable, may work in place of the MK-911 mil-surplus twinlead? Will power handling capabilities or antenna frequency ranges/wire leg length change appreciably? TU es 73,
    Dave, N8LBF

  2. I love big antennas for activations. My last activation, I used a 130foot dipole made with a Chamelion Micro transformer and worked 160 to 15 meters. It was a stretch at 160 but I made 11 contacts. I ordered my Mk-911 and look forward to building it. My two favorite wire antennas are OCF dipoles and Doublets. A doublet gives you a little DX-ability. This antenna should also pair well with the Icom AH-705. I guess I will be building more antennas this year šŸ™‚ Thanks for the post.

  3. Tom, might I suggest the “W3EDP” or the half-size “W3EDP Jr” antenna for one of your monthly antenna-builds?

    Here are my notes on the W3EDP and W3EDP Jr: http://wd8rif.com/field-antennas.htm#w3edp

    I’ve built the W3EDP the standard way using a 67′ radiator and a 17′ section of windowline as feedline. I’ve also built the W3EDP using an 84′ radiator and a separate 17′ counterpoise wire.

    Both the W3EDP and W3EDP Jr require an antenna tuner.

    (I will add that the W3EDP Jr I built in 2018 using very cheap 300-ohm twinlead didn’t age well; the twinlead broke in several places while in storage. This antenna needs to be rebuilt before I can use it again.)

    73,
    Eric

  4. What is the difference between a “doublet” and a dipole? Is there any and they are both just different names for the same thing?

      1. Thanks Eric.

        After reading that I realized that my ZS6BKW is really a type of doublet.

        I built’ Thomas’s speaker wire antenna and I think I will build a doublet to hook to my G90. The G90 is the only built in tuner capable of handling a doublet’s SWR. The IC-7300 will only handle a SWR of 3:1 or less though mine can tune around 4:1. And my Ft-891 doesn’t have a tuner so I have to use resonant antennas with it.

  5. Thomas and readers, HNY !!

    You got me reading and surfing the web for doublet info and calculators ! I was busy for an hour without noticing the time šŸ™‚ šŸ™‚
    Interesting especially when one has a trx with tuner. And so easy to build. I totally forgot about the antenna.

    Will give it a try.
    tnx and 72
    ON6UU

  6. I have a homemade crappie pole vertical. I mount it on a small piece of pvc pipe to elevate it up a few feet. I have a coil at the bottom of the antenna with taps for 20 (the pole was too short for a full length of wire for 20m), 30, and 40 and use an elevated counterpoise (linked) ala the Buddistick. I feed it with my Packtenna 1:1 balun. Works great!

  7. Great antenna article. Thanks for the encouragement.

    I am going to work on one of these soon……

    Again thanks and 73.
    Wb8yxf

  8. Hi Thomas,

    I ordered the MK-911 and I will be building a doublet off of that platform.

    Do the lines attached to the MK-911 that are being used as the twin feeder line need to be kept separated like homemade ladder line does? Is any kind of current balun or unun required?

    I am thinking that this is essentially the same as your speaker wire antenna but it doesn’t mount directly to the back of the radio. Instead feedline allows it to be placed further away.

    Just making sure I understand this setup. Thanks

    1. Hello, Marshall.

      The wire that comes with the MK-911 is actually closely-spaced twinlead feedline with a characteristic impedance of about 72 ohms. It should not need to be split or otherwise separated. Since it’s twinlead, it should be kept away from metal and shouldn’t be allowed to lie on the ground. In practice, I’ve found that I don’t need to worry much at all about either situation.

      Ideally, one would use a balanced antenna tuner with a doublet but a tuner with a built-in or external balun would work fine even if it wouldn’t be ideal.

      When I use my 44′ doublet with my KX3 and internal KXAT3 ATU, I use a small 4:1 balun between the ATU and the feedline.

      A balun isn’t strictly necessary. If your tuner can find a match without a balun, go for it!

      73,
      Eric, WD8RIF

      73,
      Eric

      1. Thanks Eric,

        Blowing up the images and examining them closer I see that it is a single line and not two separate lines like I originally thought.

        I don’t a 4:1 balun but I do have a LDG 4:1 unun so if I can’t hook up directly I’ll use the unun with a short coax jumper. It will be hooked up to my G20 so I have no worries about getting a good match.

        Thanks again.

        73

        Marshall W4MKH

  9. Just got out my doublet and am going to shorten it to 88ā€™ (thanks for posting that article in the comments!). I have my wires on some camping clothesline spools. Using some Wireman twin line to feed it. Any suggestions on feed line length?

  10. For multiband use, you want to choose a feedline length long enough to reach from your rig to the antenna feedpoint while ensuring that the length of one doublet-leg and the feedline is not an even multiple of a quarter wavelength on any band of interest.

    Generally, make the feedline a convenient length and if your tuner can’t find a good match, either shorten or lengthen the feedline a bit.

    73,
    Eric, WD8RIF

  11. I built the Norcal Doublet per their instructions. It’s now my main antenna here at the home QTH. I’ have a “No Antenna’s” restriction where I live so I put it up for a period of time then take it down. I’m using a 31′ Jackite pole to hold up the center and the two end are just attached to some high vis cord to plastic tent stakes. It works well, I’ve received several good reports. The antenna tuner is the Hendricks BLT + kit, and has no trouble tuning this antenna from 40 -10M. I also just ordered the K6ARK EFHW Kit. Really looking forward to trying it out. My rigs are the QRPLabs QCX’s (the original ones) for 40, 30 and 20M and an original FT-817.
    I really look forward to seeing what you come up with for antennas. Will be interesting to see what you find works better.
    I really like the content you are putting out there. It has inspired me to do much more portable operation this year. So Thanks!
    Good Luck, and please stay safe.
    73
    Mike
    KA7PLE

  12. I ordered the MK-911 on 01/05, the same day that Thomas posted this.

    It arrived in the mail today (01/11) and it was packaged really well. It appears to be well mage and to be New old stock.

    Now I just have to figure out how to best build the doublet. I want the antenna to be fully self contained so I will probably build it on the winder and pull it into a tree or mount on top of a walking stick as opposed to doing it Erics way on a light weight pole.

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