I started writing a sort of journal of my RF experiments on a blog. I’m far away from being RF pro, but maybe it can help, you can read my article about it at Measuring Transformer Efficiency | F4LEK - Amateur Radio Blog
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I think you have misread the specifications for Belden 88907 or there is a typo in the data. The bulk cable weight is specified as 26 lbs/1000 ft.
So a 10m length would weight about 380g.
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TANSTAAFL perfectly describes the choices when deciding which cable is better.
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All my HF SOTA antennas use RG-174. It’s light, flexible and it’s never longer than 10 mtr.
Maybe there’s a lighter or lower loss variant, but I never felt the need to investigate.
The research and analysis performed by @G3CWI to come up with an appropriate coax for the SOTABeams antenna lineup has worked very well for me, not only for my dipoles but also with my home made antennas.
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It does if the load impedance is also 75 ohms.
With a 50 ohm load, the impedance at the rig will vary between 50 ohms and 112 ohms (SWR = 2.25 : 1) depending on the cable length in wavelengths.
@9N7AC A tuner needn’t have high losses if it is well designed, and matching a reasonable impedance. (Higher impedances often have lower losses than lower ones.)
But I agree that it is convenient not to need a tuner: neither of my current QRP rigs have either a built-in tuner or even an SWR meter. That’s why I tend to prefer simple antennas that I can just set up and use, knowing that the SWR will be low enough. But, confronted by the combination of a large quantity of 75 ohm coax in my attic and the curiosity to see if I can make use of it, I decided to develop a dipole kit using it to cover 40 / 30 / 20m for my YouKits EK-1 rig. The same approach can be used for 20 / 15 / 10m coverage.
Starting with an electrical half-wave of coax for 40m was an easy choice. That works well with standard dipoles for 40m and 20m. But on 30m the coax was about 3/4 wavelength, so my antenna needed to be about 112 ohms. A full wave loop with proper dimensions would work for monoband use, but is a bit large for many portable masts.
There are two relatively easy ways to raise the feedpoint impedance of a dipole… The first is to feed it off-center. That requires a better balun, and probably would be subject to more interaction with the other wires on the same feedpoint when I want to have all bands up at the same time.
The other approach is to make the dipole longer, and add capacitors to cancel the reactance. Making each side about 8m long, with a 160pF capacitor in series with each side at the feedpoint, gave me good results for 30m in a model, although I haven’t tried it out yet. The capacitor probably needs a high value resistor in parallel to reduce static build-up.
For a conventional 20m/40m link dipole, it turns out that connecting about a 18 pF capacitor in place of each link provides a good match to 112 ohms on 30m. That’s another approach that I want to try.
Since I’ve been focused on 40/30/20m, I haven’t looked at what it would take to build a 20/17/15/12/10m set of dipoles. But it may be possibly by designing the antennas to present the required impedance to give 50 ohms at the transmitter for the chosen length of coax.
Now, I understand that not everybody is going to have appropriate types of 75 ohm coax handy to implement these techniques. But sometimes you have to improvise with what you have available, and this might help someone who only has access to 75 ohm coax, even if it isn’t for SOTA.
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1.5:1 may be perfectly fine for normal conditions, If my SWR is more than 1.3:1 then I re-tune my antenna. Normally when I am operating I aim for 1.1 or better.
According to this chart a VSWR of 1.5 represents a mismatch loss of 0.17dB, compared to 0.01dB for VSWR 1.1.
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The best way to reduce the weight of your pack is to reduce the weight of you.
Once that’s accomplished (which costs nothing), feel free to shell out on Gucci coax 
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WTF?? My gut is 93% high energy lipids. It’s a superfood, and not a gram of it is on my shoulder straps.
Do you have any idea what the missus pays for exotic superfoods and dehydrated gourmet tramping tucker? Little does she realise there is a veritable goldmine sleeping right next to her. (or maybe she does since she pays the life insurance)
You might be right about my arms. T-Rex could have been onto something there.
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That value is for 50Ω resistive source.
A transmitter is not a 50Ω resistive source, any more that the mains outlet in your house has a specific Zo.
Interesting to know what really happens to the power to the antenna if I present 75Ω to the rig?
I made an L-pad, 50:76.1Ω, -5.74dB //90Ω + 44Ω
Using my FX4 at 14.310 MHz, 11V, CW, power setting. R&S test set power meter
Set-5W P50Ω = 37.9dBm I=1.72A
Set-5W P75Ω = 37.6dBm, I=1.51A. If I turn it up the power for the same DC current
Set-7W P75Ω = 39.0dBm I=1.70A
So for the same power setting I lose 0.3dB, and for the same DC current I gain 1.1 dB i.e. the transmitter is more efficient.
Set-10W P50Ω = 40.7dBm I=2.31A
Set-10W P75Ω = 40.2dBm I=2.02A
Set-13W P75Ω = 41.1dBm I=2.30A
Above 10W the FX4 was being a bit erratic, so I stopped there for now.
So trying some more values using a different variable load with diode power meter (so the result for 50Ω 5W is slightly different
5W 25Ω 1.84A 37.4dBm
5W 50Ω 1.69A 37.4dBm
5W 100Ω 1.38A 36.2dBm
5W 150Ω 1.29A 36.2dBm
Every dB really doesn’t count. From minute to minute a DX path will typically fluctuate in the 10s of dB. 1-3dB will not have an effect on results. It is interesting from an engineering point of view, but don’t mistake it for something that will measurably increase the number of contacts.
Getting on air promptly, staying on air longer, spotting, and being able to use bands that briefly open gives you contacts. Leaving home late, equipment failures - broken wires, missing bits when you reach the summit, flat batteries, have all lost me contacts. Never dBs
Absolutely. Dynamic i.e flexing life of foil screened and solid cored coaxes is limited, they are made for static applications. When water gets in copper braid over AL foil low loss coax, it turns to blue powder in short order.
I use stranded RG174 because it goes through aluminium window gaps and door jambs in huts without being chopped off or breaking. There might be some electrical reason too.
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I’m not sure if LMR 100 is exactly the same, but DX wire offered a version of rg174 called DXW-174. It had a solid copper conductor which avoided so-called skin effects on the lower HF bands. One problem I had with it is that the inner conductor broke, as pure copper is rather weak compared to copper clad steel. It probably has a minimum bend radius that needs to be observed and is perhaps not suitable for winding chokes. I seem to remember you saying LMR 100 is very similar Andy. I notice DX Wire stopped offering DXW-174. Were there too many complaints perhaps?
73 de OE6FEG
Matt
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I was going to buy some a while back but it was out of stock. It could be there was too high a level of complaint. Or possibly the volume DX Wire had to order up front vs. the rate it sold was not economic.
There are several versions of LMR-100 some are suitable for outdoor use and some indoor only. There is one version with fibre reinforced PE jacket. It’s foil/foam cable so it will have a significantly more restrictive bending radius that RG-174/RG-316. It is lighter and much lower loss than RG-174 and so answers the original question “is there a lightweight coax with less loss than RG-174”.
The original question did not ask is it viable for use on SOTA when it will be coiled and uncoiled a lot. The answer is maybe/probably but with caveats. Its life will depend on how it is handled. So maybe it has a 1/4 the life of RG-174 and will need replacing more often. Some people replace cables when they break. Having had an RG-174 feeder go intermittent which took out an 817 PA I now replace feeders such as the RG-174 centre fed dipole feeder every 5 or so years. The cost is peanuts for the knowledge that I have taken out 5 years fatigue flexing from the failure modes.
I have a patch lead made from M&P Airborne 5 that was bought a few years back. It was used on every activation for 18 months and is still perfect. The cable was an off-cut available for pennies, the special crimp connectors cost more! Dire warnings were given but it gets handled correctly. It is wound into a coil 32cm in diameter that hangs on the back of my bag. It’s unwound and wound like a Slinky ™ and hardly ever unwound completely so the repeated flexing which damages foil outers is minimised. Is the lower loss over RG-58 noticeable? I don’t know but it was an experiment to see if using these cables was viable and I think for this purpose it is.
I’d expect LMR-100 to also be viable if treated correctly. It will need coiling into a significantly bigger diameter coil than RG-174 and done so with care. My contest group have many lengths of LDF 4-50 which requires careful handling if not to fail in the same way as foiled cables do. Nobody is allowed to coil/uncoil it if they haven’t been shown how to do it. We check the lengths once a year and after 20+ years contesting (and it was all secondhand to start with) the measured loss etc. is still within Andrew Corp. specs. RG-174’s advantage is you can treat it with less care and get away with that. Is it worth the cost and more restrictive handling? Maybe. For HF/10m probably has a measurable effect, just.
But probably greater benefits come from using an antenna with more gain. I can say that my 10m Delta loop knocks the pants off my Inv-L EFHW on 10m by a noticeable margin. I haven’t tried a 1/4wave on 10m this last year. Probably Half-Square or Bob-tails would be more effective than lower loss cable. But then you are into single band territory and having a KX+ATU+ EFHW&match or 41ft random&match gives effortless multi band capability that is viable for SOTA activators in North America or Europe. Less for more distant parts of the world.
As always YMMV.
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My experiences would seem to confirm this. I swapped mine for double braided RG-174 and double braided RG-316.
TU fur info
73
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How much does the rucksack weigh? 500g, or 2500g?
73