A Simple Field HF Antenna

Posted: 11/03/2015 in Communications

I recently had a reader email regarding his low power (QRP) field antenna utilizing the NVIS mode. He commented on his systems lack of performance.

He is using a dipole with 65’4″ legs for both the 40 and the 80 meter band. He is using a bnc-to-binding-post adapter as his “cobra head” with WD-1 field phone wire for antenna wire. His radio system consists of a Yaesu 857 and an AT-100 Pro Auto Tuner. He is trying to use NVIS to fill in the skip zone gaps in his AO.

Here are the problems that I see:

1. Trying to use one home made dipole for 2 bands.

Using the formula 234/f Mhz (for quarter wave length) and assuming the middle of the 80 meter band is 3.75 Mhz, that gives us a length of 62.5′ for each element.  So the antenna is now a half-wave dipole and is good-to-go for 80 meters.

For 40 meters that same formula results in a quarter wave length of 32.7 or 33′. He is now trying to use what amounts to a full-wave antenna for the 40 meter band. A full wave antenna is a bear to work with due to the very high impedance at the center feed point which makes them very difficult to match.  Nearly impossible to use with any coax.

2. Lack of a matching device at the antenna feed point.  Here is a picture of his feed device or “Cobra Head”. (Don’t worry dude, I removed all geo-location data from the pic). By the way, I definitely would not have any loops tied in insulated antenna wire for strain relief. You’ve built in RF chokes. Non-insulated wire – no problem.

IMG_20151027_181342_176

When you are working with NVIS, the input impedance changes with the height of the antenna above ground. From as low as 15 ohms near the ground to as high as 120 ohms when the antenna is raised. Depending on the coax such as RG-58 (5O ohms) or RG-8 (75 ohms), you will have mismatches. If you are using this system with regular long-haul comms and the antenna suspended at least 1/4 wave above the ground, it will work just fine.

The tranceiver tuner will take up a lot of the slack, but it will not reduce the losses, it just hides them from the transceiver. When working QRP you need every watt to radiate from the antenna.

Here’s how I would fix the problem.

1. Order solid or braided, non-insulated wire from thewireman.com, measure out the required maximum length for 2, 1/4 wave sections on the longest band you will use and spool each up. Hint:  There are very, very, very few resources available for 160 meter DF. Remember, you can always go shorter, but it’s hard to go longer if you don’t have enough wire in your ruck and you’re 100 miles from no-where. Cut (not a physical cut but a measure to length of wire rolled out, the rest still secured on the spool) your antenna length for the freq you will be on, using the above formula. Then spool it back up when you are done.

2. If running in NVIS mode, install a 1:1 current balun like this:

QDP7__80980.1431366363.1280.1280

You can find it here: http://www.balundesigns.com/qrp-model-1110-1-1-isolation-choke-balun-1-54-mhz/

While it won’t fix all your problems, but it will clean up your signal.

3.  Add a 30′ max length of RG-58.

4.  2 – 50′ sections of 550 paracord with a large bullet type bank fishing sinker tied to one end of each.

5.  Measure out the antenna wire from the spools and secure the remainder. Attach each free running end of the wire, as well as the coax, to the balun. Attach a section of 550 cord to the end of each spool, throw the sinkers over a tree limb and host each end up to the height desired and tie the weighted ends off. Move the antenna up and down until you hit the sweet spot with the guy on the other end.

Bottom line: If your going to drop a grand on a high end QRP rig and tuner, you really need to spend some time on the most important part of your comms system, the antenna.

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Comments
  1. Chris says:

    I run a similar dipole, about 110′ end to end (a bit short for 80m) and feed it with 1/8 wave (80 m) 300 ohm ladder line. The on 40m the line is a 1/ wave transformer. It is fed by an auto-tuner. No problems with NVIS on 80m or 40m.

  2. Chris says:

    That is a 1/4 wave transformer on 40m … Pls edit the post.

  3. shocktroop0351 says:

    Could you use this antenna for skywave and nvis? And also do you think there would be any problems for digital use? I’ve been trying to standardize on an antenna design and i like the simplicity of this one.

  4. Shocktroop0351 says:

    Another question, Which wire would you prefer from thewireman.com? I was looking at either the 511 or the 542. The flex weave sounds like a good option to me, but the higher break strength of the 511 might be nice. I usually get my butt kicked trying to get antennas into trees, so I tend to reef on it a little to get past branches and what not. I’d be interested to know what you prefer and why. Thanks again.

  5. danmorgan76 says:

    ST,

    I haven’t used the 511. I try to stick with the non-insulated wires for field use. The description online is a little vague. The wire I prefer as a field wire is the 512A @ 375lb break strength. You could go with the 512 with more strength but greater diameter, weight and it’s pretty expensive. Either will last forever. The 542 or 543 are both more fragile but you can roll a lot of it up on a spool. It’s cheaper and lighter. The great thing about any of them is if you do break it, you just tie it back together with a square knot and your back in business. I would order some short pieces to look over if you are uncertain.

    Regarding modes of operation. I use the same wire for digital, CW, NVIS or long distance. Just remember to watch your power output and SWR. The antenna in the article is my field antenna and must be recut for each different frequency used. For base station ops I prefer the Buckmaster or Carolina Windom OCF. Put it up and forget about it.

  6. Donk says:

    DM76,

    My beloved daughter lives 200 miles from me. I have a IC-703 QRP rig and am trying to set her up with a receiving station so she will know I am coming for her. I am a newbie to Ham. Can you give me guidance on what freq to set her up on? I will spare NO EXPENSE

    • danmorgan76 says:

      Where do I start? Regarding a set freq: It would be next to impossible to give you one HF frequency that would work every hour of every day, 24/7/365. HF freqs. don’t propagate in the atmosphere like UHF/VHF freqs. If you are dead set on HF, and you are not worried about expense, the best advise I can give you is to set an identical rig with more power, (at least 100 watts), and antenna up on both ends. Install your antennas for NVIS. Lots of info on the internet regarding NVIS. Then forget about voice comms and go to a very simple digital setup such as PSK-31. Lots of info. regarding PSK on the internet also.

      Now start scanning the bands for open frequencies. For 200 miles range, I would suggest the digital bands in the 80 meter band at night and the 40 meter band during the day. Sometimes 80 meters will work during the day also. Get both rigs up and running and get her on internet chat or the cell phone and send her a message. When she sees your text message, she can give you the OK over the phone. Once you’ve founds some freqs that work pretty good, try to settle on one that works all the time, as well as an alternate. As long as she is not transmitting on her rig she doesn’t need a ham ticket.

      Now the downside to this system. If you want this system to be bulletproof when the cell and landlines are down, how will you know she received your message? She can’t send you a QSL (receipt of message) unless she also has her General license and you both are on the air simultaneously.

      One answer is to go to a more complicated digital software known as RMS Express. With RMS she can leave her radio tuned to one of your pre-arranged freqs. and it will listen for your traffic 24/7/365. You send, and if her radio hears your radio, it downloads your traffic and automatically sends the QSL back to your laptop. Her radio can to this while unattended. And vice-versa. She still has to have at least a General license since her rig went into transmit mode.

      So, it all boils down to this: Is she willing and able to get her General ticket? And are you both willing to spend the time to get the rigs up and running?

      If the answer is no, then your other option might be to invest in a couple of SATPHONES. They are great if you’ve got the money. Here is where you can look them over: http://www.satphonestore.com/ One caveat, they violate my comms prime directive: Don’t rely on someone else’s infrastructure.

      Hope this helps. As usual, the readers are welcome to reply with any pertinent information.

  7. Shocktroop0351 says:

    Hey Dan, guess what? You got it, more questions. You said that you roll up your antenna legs on a spool, and unroll them to the length you need. Do you include the spooled wire as part of your measurement? Also, would the wire being insulated make a difference? (I know up above you said bare, I’m just asking so I can get a better understanding overall) I’m working with an end-fed half wave antenna (the EARCHI style) and would like to just have a spool of wire for it cut to 40M, that I can roll up for 30 or 20. Yesterday I was having a lot of luck with it on 5W (CW through the reverse beacon network) on 20M and then the band went dead. I would have liked to continue working on 40M, but I didn’t have enough time to take the antenna down and add wire. My other thought was to just have a 6″ pigtail attached to the matchbox and lugs to connect to a 20M cut of wire, then if I wanted to change bands just unhook from the matchbox and add a section of wire in to make it the length I need. I don’t think this would be as flexible or efficient as a roll of wire though. Thanks again for your advice.

  8. […] still intend to build a dipole antenna for field use per the instructions from Danmorgan76. I had a 20 meter dipole until it got snagged in a tree and the little speaker wire I was using […]

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