On the week-end I tested my new portable HF antenna – a simple doublet targeted at 40m. That is, a half wave dipole fed with parellel feeder.
My previous portable antenna was an end fed inverted V – targeted at about half a wavelength of 40m. However I was finding that this antenna was highly dependent on a good earth connection, and sometimes portable this was a struggle.
With the doublet being feed with a balanced line, it’s a bit more tolerant of poor earth and varying conditions in general. So for portable, it definitely seemed a better fit.
Further, I saw it mentioned on the VK SOTA mailing list and after a bit more reading came across the NorCal QRP Club Doublet made from some 4 strand computer ribbon cable. I didn’t have any of that lying around, but I did have a roll of some figure 8 speaker cable from Jaycar.
I was concerned about the weight of this cable (especially compared to the above mentioned ribbon), but it was on hand and something I could try pretty quickly. In the end although it’s a bit heavy, the squid pole seemed to tolerate it – but it could be good to do a lighter version and maybe use this one more in a tree.
For the dimensions I kept it simple. My squid pole is 9m, so that was the length I chose for the feeder section. Then the idea was for it to be a half wave dipole for 40m. Having two legs from the one section (once split down the middle – being figure of eight cable) I simply needed another 1/4 wavelength of cable added to the 9m.
This worked out at around about 9m plus 10.56m (aiming around 7.1MHz, but not too precise). To this, I also added 4cm to allow for folding back over of the end of each leg to form a loop for attachment. So all up, I had 9m plus 10.6m giving me 19.6m of wire to cut.
Once measured and cut, I split the 10.6 down the middle and folded back the ends and held in place with cable ties and added some support rope/string.
For the middle, I wanted to add some cable ties both to ensure it didn’t split further and also to add a larger cable tie as a support mechanism.
Finally, at the end of the feeder section I added a couple of crimp connectors to feed into the balance connection on my portable antenna tuner.
The result was good, and worked first go when setup in the backyard with no earth connection. With the parellel feeder having such a small gap, it’s only intended for QRP. But with QRP (<5W in this case, and approaching 2.5W) I happily reached NSW, Northern Victoria, Melbourne and North West Tasmania with only a short period of testing in the morning and then afternoon. (From my backyard at QE37pa.)
I also attempted tuning it on 80m (but not attempting to contact) and found that was possible, albeit with a reasonably narrow bandwidth. But that said, it was easy to tune on 40m and 80m where my previous end fed inverted V was not – unless a good earth was present.
So, a good result and I’ll definitely be using this as my portable solution going forward. Although, I may attempt to get some lighter cable and make another (maybe some simple bell cable).
Just a finishing note – Above I detail that I simply cut this antenna based on calculations rounded up for 7.1MHz and then didn’t attempt to tune and recut for best SWR. This was because seeing I plan to use it in many varying locations and always with a tuner I thought I’d just keep things simple. If it was going to be a permanent setup, I’d do it differently. But mainly, I had limited time and just wanted to give it a go. My next one I may take out to a nice flat oval and set it up and then tune before finalising cable ties etc – and may also trim the feeder to best fit based on squid pole droop.