The contenders for the 20m short-out are:
Antenna 1: A homebrew delta loop, corner fed (vertical polarisation), made of kevlar reinforced wire and connected via 15m of RG213 coax to a G-Whip 4:1 balun. Apex height 8m, base wire at 1.8m, antenna at right angles (peak gain) to the E-W line. Total cost: wire £22, balun £45 = £67.
Antenna 2: A G-Whip end-fed half wave, also of kevlar reinforced wire connected to a matching unit via 15m of RG58 coax. Mounted as a 45 degree sloper to the west from 8m to 1.5m above ground, feed point at lower end. RRP: £69.
A stranger at the edge of town - a figure-of-eight small copper loop also enters into the fight later on.
8m was chosen as a maximum height because it is about the most you can get out of 10m fishing poles that are likely to be used whilst also being able to support any weight or take any strain.
Firstly, there is obviously an immediate, if slight disadvantage to the EFHW, because it's connected using bog-standard, cheap coax that has higher losses than the RG213, but only by about 0.5 - 0.75dB, if you believe standard published charts.
How did they do?
Firstly, my subjective assessement, aided by the 'S' meter, was firmly that the EFHW came in significantly weaker on receive by 1-2 'S' units, or 6-12dB if my meter is anything to go by. Ignoring the meter, I could hear very weak stations to the point of definite readability on the delta, but only knew the station was there and could not make out what was being said on the EFHW. I did not have time to run lots of transmit cycles, so I can't say anything about that, but from this location at this moment, the EFHW was easily weaker, and significantly so, than the delta on receive.
Then, I ran some quick WSPR tests over a period of about an hour between 07:00 and 08:00UT. I chose a brief period, switching for 10 minute periods between each antenna, to lessen the effects of transient propagation effects and progressive changes as the sun climbed in the morning sky.
This revealed a rather more complex picture. Prior to the onset of LP propagation, both the delta and the EFHW were getting out in physical terms to much the same kind of distance, to Israel or thereabouts from Wales. Signal comparisons were, as received at 4X1RF (in a direction away from the EFHW's slope and peak gain)
Delta loop: mean of -10.8dB (+\- 2.6dB) EFHW: mean of -23.5dB (+/-0.7dB)
For ZL4VV on long path, where signal attenuation is vastly reduced compared to other paths, the difference was hugely less, and well within the QSB variations, although I could only obtain one reading due to lack of time to do any more:
Delta loop: -20dB EFHW: -22dB
Now, don't read too much into those numbers. WSPR is a weak signal mode that involves miniscule signal strengths that could never be heard by a human. All you should really take away from the WSPR test is that the EFHW is weaker than the delta, but seems to be comparable to a delta on long path, but that there are discernible differences in signals using CW and SSB on some paths. The earlier SSB receive-only test confirms this semi-subjective, semi-objective assessment as the delta being the clear winner. Given the delta has a large capture area, this is hardly surprising.
Going Loopy!
Now, we may forgive the EFHW for not getting across as well as a wave-catching delta loop. But can we be so forgiving when it fails to do as well as a tiny copper double loop antenna? My colleague G3JKF is a keen and careful antenna experimenter using WSPR data to guide his design. I ran the EFHW against Ken's double loop during a stormy night in January 2013 and the following morning, when it was just as windy. Ken's loop, mounted low on a fence post, with a flower pot to cover the capacitor, and which would stand up to any hurricane with ease, was consistently beating the EFHW on both short and long paths. You can find details of a similar kind of loop here.
So that I can't be accused of not presenting any data, it's all there for you to see on the WSPR spot database, but here is a random selection from around 10-11:00UT on 27/01/2013, with the EFHW sloping to the west.
To R4HAC - EFHW -22dB, G3JKF loop: -11dB
To OH5CW - EFHW: -8dB, G3JKF loop: +3dB
To VK5ZTS - EFHW: -26dB, G3JKF loop: -22dB
To IK2RFI - EFHW: -26dB, G3JKF loop: -4dB
To W4AC - EFHW: no signal received, G3JKF loop: -20dB
I was going to highlight the fact that the EFHW is just a single wire, very easy to put up in a hurry, and can use trees or similar to prop up the upper end. But then, a delta is not much more complex, and I have put one up in a howling gale on a beach and in the surf in a few minutes. And whilst building and tuning a small magnetic loop is not very easy for the uninitiated, it's obviously infinitely easier to mount and poses few problems in transport even if you are on a bike!
Also of importance to the very many of us who live in cramped surroundings is that an end-fed can be put up and pulled down on a fishing pole in seconds and is a very easy, practical way to get an efficient antenna (about 90%) on the go. Planners and neighbours would have to be especially persistent and unreasonable to object to an antenna of this nature. But again, the small loop wins hands down when it comes to these matters; due to its size, it will probably escape most people's notice, won't be recognised as an antenna by many, and it almost certainly wouldn't ever form the subject of any planning enforcement.
No comments:
Post a Comment