I built the 22ft doublet like described on the sotabeams website:
But I can’t tune it. My mast is 6m for portable. Intentionally I wanted to build it for my 12m spiderbeam. But in the end I decided that its better to use it as a portable solution on SOTA activations.
Anyway, looking at the doppelzepp calculator which is posted above, the 22ft are 6.705m and are not suitable for 40m…
Going through the numbers to get everything in the green, I end up with 8m of legs and almost 9m of a open wire feeder. 9m is quite long and pretty unhandy in the field. Especially I’ll have to lay it on the ground. Does it affect performance when laid on the ground?
What are your feeder lengths for portable use?
Because for Sota it is nice to have a variety of antennas available, I also have a Doppelzepp.
Try the follwoing configuration and look at the Verkürzungsfaktor. I use a thin twin lead cable where the leads are 2.5 mm apart (center to center). This gives a Vk of 0.59, which was measured using a Network Analyser. Then you end up with the following results (the antenna of course is too short for 160m and 80m):
I question using that red cable as feed line if that’s what you plan. It will work but you will have several dB extra loss (high SWR along feed line for this type of antenna). I would recommend 300-ohm TV type twin lead, 450-ohm ladder line, or PTFE insulated twisted pair salvaged from CAT5E or CAT6 ethernet cable.
I am using homemade ladderline with 28mm spacing. According to the spreadsheet the VF should be around 0.97. So my question still remains, what lengths do you guys use for the open wire feeder, since the calculated length seems to long for me
I use the shortest length that will reach the radio. I don’t understand why anyone would say the feedline length is critical or needs to be a certain length.
Feedline length apparently influences impedance and therefore the tuning. I cant tune the antenna on cetrain bands with my shortest feedline length. I shortened it so it does not run on the ground and since then I am not able to tune it right.
Yes, this can get very complicated. The feedpoint impedance is transformed to some value at the tuner terminals by the type and length of transmission line, and differs widely by band. It’s obviously been transformed on one or more bands to a value that the tuner can’t match. So some length of transmission line must be selected that will result in a matchable impedance at the tuner. This is difficult to solve mathematically (many variables, Smith charts) and usually solved by trial and error variation of the transmission line length. Keep the line away from ground and metal. Alternately, try a different tuner. Sorry for not being of more help.
I use the zs6bkw variant of the G5RV multiband dipole/doublet. It is 24m metres long (end to end), fed in the centre at about 6.5m above ground, by 11.1m of 300 ohm tv ribbon from my junkbox. It works fine on all bands except 15m and 30m, though on 30m the kx3 atu does handle the high impedance. I’ve worked long path from VK to EU countries with it, and also my only S2S with NA was made with this antenna on 17m.
As the feedline is much longer than the mast height. I route the feedline down to the 1m level where the guy ropes stabilise the pole, then it comes across to the operating position. I position the radio about 4m away from the base of the pole. If bushes or trees are available I route the feedline over small branches or rocks to keep it off the ground. If no bushes or rocks are there I use my backpack. And if some feedline still lies on the ground, I don’t worry about it. Portable operation and making do with the available materials requires pragmatism, so the idealist is always going to be disappointed.
The short doublet shown in Richard’s video (sotabeams video series on youtube) is great but it relies on a wide range ATU to adjust the impedance for the radio. Many simple ATUs are ok to match a low SWR like up to 5, but for 2000-3000 ohms reactive that some lengths and wavelengths produce, more inductance and more capacitance range is needed on the antenna side of the matchbox. Not many commercial matchboxes that you want to carry in your pack actually do that.
Struggling to avoid antennas other than simple resonant centre fed antennas just to avoid using coax is in my view, not worth the trouble. I’ve used RG58 coax for the SOTA antennas since 2013 and it has never been too heavy to lift, carry or for the pole to support. I really don’t know what the problem is. I only went to the zs6bkw to achieve quick band changes having seen how quickly others using this antenna could do it. But I still have the other proven antennas like the linked dipole and the quarter wave vertical with elevated radials. They all work well.
I have a linked dipole too. That DXW174 looks promising. When having a lot of homebrew antennas in the portofolio, it’s always a struggle to choose what to take on a trip
Hi,
The link dipole is the gold standard and an ATU is not required. But it is slow to change bands. At home I use a 5 MHz doublet fed with home brew open wire line. It will tune 80 thru to 10 m but the pattern is complex above 20 m. I use it for 80, 40, 30.
So for SOTA I now have a 10 MHz/5 MHz link doublet with 300 ohm ribbon feeder. For 80 - 20 m the link is closed and for 40- 10 m the links are open. I use a current balun to an LDG tuner
If I expect the higher bands to be open I open the links before raising the antenna otherwise they remain closed. This allows good operation on the lower bands but with compromised pattern on the higher ones.
So long as the radiator is not less than 70% of a half wave it will be effective. Once it exceeds 3/2 wavelengths the pattern has too many notches IMO. High SWR is not bad if the tuner can cope and the feed line length is modest. Radiator lengths outside the range above may call for the tuner to match very low resistances which is hard to do efficiently.
I tried new lengths. 7.9m leg and 7m feeder. I was able to tune it even with my qrp tuner from qrp guys. It was very very touchy to tune though.
I like it, especially because the poletop is not under such stress. I wanted to compare it to a linked dipole with WSPR, but I ran out of battery…
