Coax for Q100 - lots of questions....

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radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

Coax for Q100 - lots of questions....

Post by radiogareth » Wed Nov 18, 2020 2:57 pm

Having finally got my Q100 station up and running I was quick to find shortcomings and so attempted upgrades.
Working back from the POTY on my dish, I have 1m of RG402 (90 degree SMA to N plug) that connects to a piece of 4-50 heliax. That makes it to the shack where I have an N-type joiner, N-to-BNC adaptor and then a 8" BNC-BNC coax patch lead salvaged from a UHF diplexer (silver plated core, PFTE dielectric, silvered braid OD 3.8mm. Then a BNC to SMA on the Stealth amp. The 8" pigtail gets hot in a few minutes. The amp is running about 50 watts DVBS2.
I've another piece of salvaged coax, from a military 4CX250 amp, conveniently ended in a N-male which will remove one N joiner and a N to BNC adapter - worth hile gain I'd expect. I can fit a BNC on the other end. It appears to be silver plated too, brown PTFE-like - slippy outer covering, 4.5mm diameter. Very stiff, so maybe silver plated steel core?
I'd assume this will be better that the braided jumper, but if not, what are people using for short patches? I have some rigid PTFE but it's also 3.8 diameter and NOT silver plated so I can't see that helping.
Finally, I have some nice 300mm Suhner RG223U SMA to SMA, I could cut one end off and fit an N male.
So some choices.....what are people using? Heat = loss which I'd clearly like to remove if possible. Its not the connectors getting warm.
Thanks
Gareth G4XAT

**** Update***
Found a SMA to N male adaptor so put the whole Suhner patch in line. Still gets warm, but not as warm. Also 3 less joints in this short part now, maybe 0.1 gain. I've nothing accurate enough to check exactly.....

g0mjw
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Joined: Sat Sep 20, 2014 9:15 am

Re: Coax for Q100 - lots of questions....

Post by g0mjw » Wed Nov 18, 2020 3:19 pm

Too many transitions. BNCs don't work well at 2.4GHz. TNCs are better.

I have a 6" piece of 0.141" semi-flexible from the Stealth PA. Right angle SMA on one end, N female on the other. Then a 4m length of FSJ4-50 with N males on each end, then another identical semi flexible from the Heliax to the feed. The semi-flex gets warm at high power but the FSJ4-50 doesn't. I think the loss is around a dB.

Try measuring the power at the feed vs the power at the amp. It can be done at low power or with a VNA. See what you are losing. Miss-match at transitions is an issue but very frequency / length dependent.

Mike

g8gtz
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Joined: Wed Apr 07, 2010 6:26 pm

Re: Coax for Q100 - lots of questions....

Post by g8gtz » Wed Nov 18, 2020 3:28 pm

Hi Gareth

First of all congratulations and look forward to seeing you on Thursday night!

Without accurate power measurement you can't really tell but you would be amazed how much power you lose in connectors, adapters, right angle adapters which should be avoided and what looks like good co-ax.

Not sure how long your piece of 4-50 but there is a reason why most people have their power amplifiers outside under the dish and just a very short lead to the feed - even so I have burnt out what looked like a good SMS to SMA fly lead with just 50 watts of power. To get the amp outside should be your next aim!

Silver braided co-ax is normally good, BNCs are not good at 2.3 GHz and semi rigid is normally good as it has a PTFE inner.

So keep it short and avoid adapters at all costs are the rule.

Not sure if this helps much.

Noel

g0mjw
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Joined: Sat Sep 20, 2014 9:15 am

Re: Coax for Q100 - lots of questions....

Post by g0mjw » Wed Nov 18, 2020 3:41 pm

LDF4-50 has a very low loss at 2.4 GHz, its about 0.13dB/m so even quite long lengths are viable. Where the power gets lost is in the connections and when the cable is not good, for example it's got water in it or has been coiled up or bent one or more times to many.

I find this table of cable losses useful from time to time.
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g8gtz
Posts: 1332
Joined: Wed Apr 07, 2010 6:26 pm

Re: Coax for Q100 - lots of questions....

Post by g8gtz » Wed Nov 18, 2020 3:56 pm

Useful chart Mike but folks should be aware that it represents the cable as manufacturered - even second hand cable that looks fine may have got damp or damaged (as I found out a few months ago).

Plus it's unlikley you will acheive those numbers in practice after you've put connectors on the end, particularly 2nd hand ones from a rally or nice shiny Chinese ones off ebay!

Noel

radiogareth
Posts: 585
Joined: Wed Jan 06, 2016 9:46 am

Re: Coax for Q100 - lots of questions....

Post by radiogareth » Wed Nov 18, 2020 5:21 pm

Thanks for the pointers :-)
Its not a very long run of 4-50. I measured it as 10m and for the moment the system 'works'. A bigger amp is in the 'works' but if 333ks is making it that's fine for the time being.
Just sorting out the shack - re-arrangement to accommodate TV and clear away 3+ years of bits/scraps/etc from building stuff!!

2 days so far.......

Gareth

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