I’ve been generally happy with the performance of my 5 band trapped EFHW antenna and have used it on many SOTA activations. However, every time I use it, I have to be quite careful not to break any of the joints to traps or get the traps snagged in trees etc. Consequently, I’ve been looking around for a simpler antenna that will still achieve multi-band coverage.
I came across the following antenna which covers the 80-10m bands with no traps and just one loading coil. It is called the EFHW-8010. It is simply a half wave end-fed on 80m which will resonate on all multiples of that frequency, loaded with a small inductance near the feed end. This inductance has the effect of bring the resonances a bit lower, particularly on the higher bands and thus improving the match.
The commercially available version of this antenna is intended for permanent installation and is way too large and heavy for portable ops. So I decided to model it using 4nec2 and re-design it optimised for portable use. The NEC2 model is in the appendix for anyone who would like to play with it further.
To see the effect of the coil, here are two modeled versions of the same length and same configuration (low inverted-V), the first without any coil and the second with a 4.5uH coil 2m from the feed point:
You can see that the SWR (measured against a 3000 ohm characteristic impedance to model the feed transformer) dips in the wire with the coil are lower in frequency and generally closer to the amateur bands. Note that the wire was trimmed to be close to 7.0 MHz full wave resonance to give the best result in the lower part of that band.
You’ll see that the SWR is low for the majority of the bands, but is probably not low enough to use without some sort of ATU. However, by getting it reasonably low, there will be little loss on the feed line and the internal ATU on rigs such as the KX2/KX3 will have no trouble matching it. The variable SWR is the trade-off for having a very simple antenna.
The radiation pattern on each band follows the expected shapes for low antennas with increasing length. Here are some sample plots for reference:
Essentially, these are NVIS patterns, becoming slightly more directional on the higher bands.
The construction of this antenna is very straight forward. The wire used was 22 AWG PVC coated hookup wire.
The inductor was wound on a 22mm PVC former (a piece of 20mm pipe) with 17 turns of the antenna wire located 2m from the feedpoint. The coil was held in place by drilling two holes in the former about 30mm apart and passing the antenna wire through them from the inside. The whole coil was then covered in heatshrink tubing to hold the turns in place. The final coil measured 4.45uH.
The wire was cut to 40m length and then trimmed when set up as an inverted V to minimise the SWR at 7.090 MHz (you could of course optimise for any preferred frequency). In my case, it ended up at 39.21m, but this will vary depending on the specifics of the setup etc.
I’ve followed my usual practice of attaching Zing-It line to each end of the wire by feeding the wire into the core of the Zing-It and using Araldite (glue) to secure it in place. This makes it fairly easy to rig in the field in an inverted V or other configuration.
This is an end-fed antenna and hence has a very high feed point impedance that will likely be somewhere between 2000 and 6000 ohms depending on the set up configuration and local ground conditions. I use the same match box that I have used for my other end-fed antennas, the details of which are here.
To evaluate the antenna a little further, I set it up in a park environment and measured its SWR across 3.5 – 30 MHz using my N2PK VNA. I made measurements using both a small squid pole with the apex around 4m above ground and also with my large squid pole with the apex at around 8m above ground. In each case the end of the antenna were approximately 1m above ground. This closely matched the topology of the earlier NEC models as far as possible.
As you can see, the measured response is a reasonable fit to the model.
It is interesting to compare the response using the two different apex heights. As you can see from the chart above, the taller pole (with a necessarily sharper apex angle) results in a slightly higher frequency for each of the SWR dips with slightly better SWR on the higher bands and slightly worse SWR on the lower bands. This reinforces the fact the the setup of portable antennas has a significant effect on the actual SWR encountered.
I have now used this antenna for quite a few activations and found that it works well in conjunction with the built in ATU in a KX3/KX2. It is a long antenna, but this is necessary if you want something that will work reasonably on the 80m band. It also gets a lot of wire in the air which can help with receive sensitivity a bit. By using relatively thin wire, the resulting antenna can be wound on a kite winder and not be too bulky in a pack.
Appendix – NEC2 model
This model uses 22AWG PVC coated hookup wire and is designed as an inverted V with the apex about 4m above the ground and ends about 1m above the ground. To make sense of the SWR analysis, you need to set the characteristic impedance of the system to 3000 ohms as this is roughly what the matching transformer will map to.
CM End fed multi-band using a single coil to adjust resonance
SY L1=2.0 ‘Length from start to coil
SY Lcoil=0.03 ‘Length of coil
SY L2=39.21 ‘Length of whole antenna
SY L=4.45e-6 ‘Coil inductance (4.45e-6)
SY EndHeight=1.0 ‘Height of ends above ground
SY ApexHeight=4.0 ‘Height of apex above ground
SY PoleX=sqr((L2/2)^2-(ApexHeight-EndHeight)^2) ‘X coord of support pole
SY Theta=atn((ApexHeight-EndHeight)/PoleX) ‘Angle antenna makes with EndHeight plane
SY CoilSX=L1*cos(Theta) ‘X position of coil start
SY CoilSZ=L1*sin(Theta)+EndHeight ‘Z position of coil start
SY CoilEX=Lcoil*cos(Theta) + CoilSX ‘X position of coil end
SY CoilEZ=Lcoil*sin(Theta)+ CoilSZ ‘Z position of coil end
GW 1 10 0 0 EndHeight CoilSX 0 CoilSZ 0.00025 ‘Section from start to coil
GW 2 1 CoilSX 0 CoilSZ CoilEX 0 CoilEZ 0.00025 ‘Coil
GW 3 100 CoilEX 0 CoilEZ PoleX 0 ApexHeight 0.00025 ‘Section from coil to apex
GW 10 1 0 0 EndHeight-0.1 0 0 EndHeight 0.00025 ‘Feedpoint
GW 4 100 PoleX 0 ApexHeight 2*PoleX 0 EndHeight 0.00025 ‘Section from apex to end
GW 5 10 0 0 0 0 0 EndHeight-0.1 0.00025 ‘Ground connection
LD 7 0 0 0 4.5 0.00039 ’22AWG PVC hookup wire
LD 0 2 1 1 0.1 L 0 ‘Coil
GN 2 0 0 0 4 0.003
EX 0 10 1 0 1.0 0 0
FR 0 0 0 0 3.6 0