EFHW observations and radiation pattern question

Started by dylan, August 05, 2019, 06:02:23 pm

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dylan

Hi everyone. I had a recent opportunity to take an MTR5b and 40m EFHW setup deep into the wilderness. Similar setup to last year's "radio vacation" (http://radiopreppers.com/index.php?topic=1482.msg13653#msg13653). The 40m EFHW was tuned via QRPGuys EFHW tuner, with driven element / counterpoise lengths from their recommendations. I was able to get the high end of the antenna up about 40 feet thanks to some conveniently placed trees and topography. Sounds like a great setup, right?

I found that I could "hear" stations all over the US at 0200 UTC (local time ~ 7pm) but could not make contact with a station about 50 miles away that was listening/transmitting. I used VOCAP (http://www.voacap.com/) to estimate the potential for said contact, and the predictions left me confident (see attached forecast). It seemed like 40m at 0200 UTC would be within the realm of possibility when using 1W CW on my end, and 15W on the other end. Note that the other station was using a 40m doublet at about 20 feet elevation, north-south orientation. I was north-east of the other station.

A couple of observations and questions for the group.


  • The EFHW + tuner combination seems to do well when the tuner is elevated at least 3-4 feet off the ground, any closer and it is difficult to achieve and SWR less than 2:1.

  • What kind of theoretical radiation pattern can one expect from an EFHF? I recognize that there are over 1,000 answers depending on the local conditions, but could there be any rules of thumb to go by? For example, one can expect maximum signal strength in the horizontal plane at 90 degree angles to the orientation of a 1/4 wave dipole at resonance.

  • Was the skip distance under those conditions / antenna the likely problem (overshoot) or antenna directionality?

  • Is it possible to achieve NVIZ-type communications with a EFHF?

  • Are gound-losses too high for NVIZ to function properly at < 5W of power?



Overall, I'd bring the QRP kit (MTR5b, 40m EFHF, QRPGuys tuner, mini-paddle, ear buds, 9V battery) on my next backpacking trip as it contributed very little weight.

I'll have it out again next week in case anyone would like to schedule a QSO on 20/40m.

73,
Dylan




vwflyer

Hi Dylon,
You have a lot of good questions. I'll try to answer of few of them. I'm not an expert but I can pass on what i think I've learned. I give no guarantee of the accuracy of my answers.

It sounds like you have a good setup. I have found my MTR3b to be a great performing little radio and I have found QRP in general to be very effective. I've also used end fed antennas almost exclusively with QRP and they have worked well for me. That being said, QRP is a lot like fishing, if you don't stack the odds in your favor you may get a bite and you may not. Calling in the blind is the least optimum way of securing a QSO. Some days you snag a good one and some days you get nothing. When you are testing out a new antenna or radio it can be disheartening when your first trip out returns only minos. You have a great SOTA setup. If you self spot a SOTA or POTA activation you will stack the odds in your favor by bringing the fish to you. Now you are being chased by others who would have not know you were there or wouldn't have been interested in a contact with you otherwise. I have gone out on consecutive trips outdoors with my MTR and not had one bite. I started to wonder if my setup works. Then I go activate a summit (spotting myself) and in 20 minutes make 30 contacts nationwide using the same setup.

To address some of your questions specifically:
Half wave wire antennas when in free space (or when more than half a wavelength above the ground) radiate mostly broadside. They also radiate off the ends of the wire but not as much. As the wire is lowered below half wavelength to the ground the horizontal radiation pattern turn more and more omnidirectional. A 40 meter half wave antenna only 20 feet off the ground will have a mostly omnidirectional radiation pattern. As for the takeoff angle, it will be quite high and NVIS will work if NVIS conditions exist for 40 meters at the time and location of your activation. Ground losses will not be too high for NVIS if NVIS  is available. I have used NVIS very effectively with 3 watts. Your takeoff angle will be high, as I said earlier, but it will be low enough for a communication range of a few hundred miles in average conditions. I have found midday 40 meters to usually work well between 300 and 600 miles with my end fed antennas and QRP CW.