OK - I spend a lot of time on the summits… and the beloved EFHWs (40/20/15/10m) are a bit older… but I was still surprised at how much the wires have stretched.
When I’m out and about with the KX2, pressing the ATU button is standard… no matter which antenna I use. I have my own resonant antennas for the QRP devices (Venus, QCX Mini, etc.).
As part of the 10m challenge, I sometimes take a small PA with me… and recently I had a SWR meter with me again. The SWR values amazed me. Somehow I remembered them better.
Today I checked my two EFHW (0.14mm² and 0.22 mm² - wire from SOTABEAMS) with the NANO VNA and saw how out of alignment the curves were. I shortened the thin wire by about 15 cm and the thicker wire by about 7 cm. I will cut new wires soon.
Maybe I should attach the NANO VNA every now and then… especially when I have to pull a little harder to get the wire out of the tree.
The Sotabeams wire is basically hookup wire and is probably not designed with mechanical properties as a priority. The strands are probably ETP Cu which has a fairly good tensile strength but only a modest yield strength. It’s not going to break too easily but it will stretch a fair bit before it does. Something with reinforcement like DX-Wire UL will be better keeping SWR’s stable, but for ‘random’ wires, the Sotabeams wire is fine. It’s cheap as chips too - just a pity they don’t seem to be delivering to the EU at the moment.
Here in the UK we have a problem with not always being able to buy from EU since Brexit. Maybe get a colleague in the North to order it for delivery there and then you can meet up for a joint activation and collect the wire.
These are the characteristics that I look for in my SOTA antenna wires… and flexible and light… that’s why I usually use 0.14 mm² wires… and in yellow, so that I can find them in the undergrowth.
It’s actually only natural that such wires stretch when you activate them over 500 times.
I noticed this afternoon that the SWR on 20m was a bit larger than usual - and seemed to be greater at the shorter wavelengths. As you suggest, Armin, time to get the Nano VNA out again.
73,
Rod
You raise a good point for us all to take note of. We forget some of the old learning.
Antenna wire used to be hard drawn copper of single strand about 2 mm diameter or thicker. It didn’t stretch. It had low losses. It was springy and could only be coiled for transit. Not good for SOTA although my first two or three SOTA activations used such an antenna.
Using insulated stranded wire means two new effects. The insulation adds capacitance and lowers the resonance. The stranding will straighten a little under tension before any ductile length increases occur. The wire will get longer over time due to both effects.
The losses will be higher at the higher end of HF due to both insulation losses and thinner conductor loss. Convenience has a cost.
For most of us the need to occasionally adjust an antenna is a minor inconvenience and the extra losses are unnoticeable.
Antenna wires for tensile strength are made with copper clad steel. However, those are overkill for temporary antennas for SOTA.
Among pure copper stranded wires, I find the jacket material matters. PTFE or FEP jacket (teflon jacket) adds strength to the wire, and the wire slides easily when caught in tree branches/leaves. PVC seems to pass the mechanical stress to the copper core more directly. PVC is also stiff during winter and does not give good hand. Silicone jacketed wires are more fragile, and also difficult to retrieve when caught in trees due to grippy surface.
I use FEP jacketed wires for radiators, and silicone jacketd wires for radial wires. Radial wires are always placed within my arm’s reach, and the grippy surface makes it easy to deploy, because the wire stays in place as I lay it on top of the brush, branch, etc. I may give one half hitch knot at most as needed.
The wire jackets for antenna and radial are not in a transmission line, so there is no need to worry about velocity factor or dielectric loss. (However, the wire diameter affects the minimum SWR frequency as well as the bandwidth.) On the contrary, when winding transformers or using as a feedline, I would avoid PVC jacketed wires, as they have 10x higher dielectric loss of PTFE, FEP and PE. (AC power cords, which use PVC jackets, are a poor choice for a feedline even in HF, despite it is somewhat popular. In fact, scrap CAT 5e or better ethernet cables make better feedline.)
The Sotabeams wire is PVC coated but is very good for portable use. It’s probably the most widely used wire for SOTA on this side of the pond (?). From a mechanical point of view a PE coating (e.g. DX-Wire UL) will give better abrasion resistance and has a more slippery feel which seems to snag less.
There is no question PVC is the most common material for general purpose wire jackets. It’s cheap and readily available. Most wires are made for 50/60Hz or audio frequencies, and PVC is good for those, unless durability under high temperature or high friction conditions are required. For example, aerospace applications use a lot of FEP/PTFE wires. Some of those surpluses show up on eBay. There are plenty of options on AliExpress as well.
Like anything else in engineering, the choice is a trade-off.
To other readers: please read this in conjunction with my previous post. The dielectric loss is not a question for antenna radiator or radials. Mechanical properties are the key factors. But to add to this point, I don’t think any serious manufacturer uses PVC for the inner insulator for high resolution video, USB2 and up, Ethernet, Firewire, etc. or any RF coax cables. It is usually PE. PE has good dielectric properties at RF but it melts at a lower temperature; soldering must be done with care.
I can only recommend the UL from DX-Wire. Very light, extremely tear-resistant and still like new and the same length after hundreds of activations. Antenna strand made of Vectran® fiber as well 36 x 0.1mm, tinned copper Cross section approx. 0.3 mm2 Insulation made of polyethylene (PE), black Total diameter 1.6mm Weight approx. 4 g/m Breaking load approx. 75 kg
