I went away to the Tasmania’s West Coast in October 2016 and had hoped to work some stations. For radio, the trip was a bit of a disaster with the place we were staying having lots of solar inverter noise. The other thing was that it was very tedious to use a linked dipole in the location I was in. Since that day I had resolved that I would try and end-fed antenna and see how that goes.
The End Fed Half Wave (EFHW) antenna is popular with many QRP Portable stations, particularly for activities such as SOTA or WWFF. The big drawback of the end-fed is the high impedances they have, which means that you need to carry some sort of Antenna tuner to match the impedance with the radio. The thought of lugging around an antenna tuner and then having to retune every time I switched frequency was less than appealing though.
Building a Matching Unit
I was however willing to compromise with an easy way to tune and started investigating the use of L Network matching units, largely spurred on by the books, videos and website of well known Australian portable operator Peter VK3YE and was just about to make one, when I came across a blog post by VK3IL on an EFHW Matching Unit.
The unit is effectively a 1:64 transformer wound on a toroid, with the idea being it will bring high impedances down towards a closer match to 50 ohms, with a capacitor used to electrically shorten the Antenna at the higher frequencies. With most of the components sitting in my workshop, I assembled the matching unit per the pictures and documentation, but notably I did not have a FT-140-43 toroid so used a Jaycar LO1238 L15 toroid instead – we’ll get to this later.
With the matching unit complete I measured out the EFHW wire to approx 20.2m for the lower band of around 7.100MHz, before and then attached to the matching unit, which was connected to an antenna analyser and set up the antenna as a sloper for testing. The results surprised me:
|Frequency MHz||VSWR||Impedance (R)|
Tuning either side of this made an insignificant amount of difference to the impedance, which is what I would expect given the 1:64 transformer. It looks like I suddenly have a no-tune QRP antenna!
So far I have taken the the antenna half way up Mount Direction as part of a failed SOTA activation (The track I was following was not the proper track and petered out to bush bashing a steep and slippery slope, which was asking for trouble!), where I discovered that I had made a cold solder joint which caused SWR to be too high to safely transmit.
Second attempt was much closer to home by climbing Natone Hill. Here I was thrilled to work JH1CDR on 14.215Mhz with a Rx 4/1 and Tx 5/9 signal report, running 5 watts.
The third outing was to Mount Rumney which was an easy to get to SOTA location, however was unable to activate as a SOTA summit due to some unexpected problems that resulted with me operating too close to my car. I was able to make the following contacts:
|Callsign||Signal Report Rx||Signal Report tx|
|VK7ZMS||– (test)||– (test)|
What was surprising about these contacts though were that due to me hitting a button on my radio at some point, they were made using only 0.5W and not the 5W I thought I was using. I was particularly pleased about these reports based on this revelation.
It’s fair to say that operating a EFHW when portable is different to using a linked dipole. some of the things I have found include:
- Your coax to the matching box should be long enough to comfortably sit or stand a metre or so away from the antenna. This allows you to better position your EFHW, along with making it more comfortable to operate.
- Always carry plenty of string with you! I like bricklayer’s line due to it’s high visibility. You use this to help tie off the matching unit end of the antenna, and sometimes that tree you want to tie off to is just 1m too far away 😉
- A corollary from the last point is to make sure you have a tent peg or two with you to tie he matching unit to if no tree available.
What About that Toroid?
As mentioned earlier, I used a Jaycar L15 LO1238 ferrite toroid and as it turns out there is very little information available. Searching around the Internet, I found articles by the well respected Owen Duffy VK2OMD. I reached out to Owen seeking more information about the toroid and he graciously obliged. One of the key things he pointed out was that the spec sheet provided with the toroid was “rubbish” and it did have some comparable permeability characteristics to the FT-140-43 toroid more commonly used:
Permiability Charts Supplied by Owen Duffy VK2OMD. Click on each Image to Enlarge
Type 43 Mix on left, L15 Mix on right
Owen has written quite a few technical articles around EFHWs and I have linked to them below (including one based on my correspondence here).
Section Edited 11/1/17 – Following email clarification Owen I have edited this section, removing comments around the L15 mix being more lossy (I misiunterpreted reading a page about magnetising admittance). The two mixes are comparable and the losses assist with the broadband nature that people seek with this antenna. Thanks Owen for clarifying – I’m still very much with my training wheels on when it comes to understanding ferrites! 🙂
I can now see why the EFHW has quite a following with portable QRP operators. It seems a relatively capable antenna and with the use of a matching unit or unun becomes a multiband “no-tune” antenna.
Operationally, considerations must be made for placement of the antenna, particularly the securing of the matching unit to a nearby fastening, to ensure ease of operation and strain on coaxial feedline.
This antenna with it’s modest performance and ease of construction is definitely worth having in any portable QRP station kitbag.
Theres a lot of useful information I found online while researching and building my EFHW Antenna. I have included some of those articles below.
- Owen Duffy – End fed matching – VK3IL design on LO1238
- Owen Duffy – End fed matching – analysis of VK3IL’s measurements
- Owen Duffy – The magic of End Fed Half Waves (EFHW)
- Owen Duffy – End Fed Half Wave / Inverted L / Half Wave Dipole
- VK3IL – EFHW Matching Unit
- QRZ Forums – Balun Measurements (discussion on LO1238 toroid)
- rec.radio.amateur.antenna Newsgroup – More Discussion on LO1238