One of the best homebrew tips passed on to me (by Dave g3zxx) was using open ended lengths of coax stub as a capacitor in single band impedance matching (parallel tapped coil and cap). It's the nature of coax to take high voltage without arcing and therefore several KW of RF power, so it out performs a air spaced tuning capacitor in both performance and cost. Once tuned the 'variable' isn't needed it's just handy in the initial set up so would be a waste of an expensive capacitor. Multi banding could even be achieved by attaching different pre tuned length of coax stub maybe via quick change BNC connector.

I am also interested in making a variable capacitor from coax, or at least one with a little tuning range to help in the initial set up so am researching that for future projects. Thinking along the lines of coax inside a sliding copper pipe sleeve. As it stands once the stub is trimmed near the required resonant frequency a little fine tuning can be had by removing about an inch of the insulator sleeve at the end of stub and bunching the braid backwards and forwards to find the sweet spot - then taping up to secure.

This post is something for me to refer to edit and add to in future...

"RG-213 equals about 50 Pf per foot of capacitace, which means every inch of coax is worth about 100-200 KHZ"

I measured Approx 15 foot of rg316 coax = approx 600pF which sort fits in with the above statement.

Longer the stub = greater capacitance = lower the frequency, trim stub to raise the resonant frequency.

It's been awhile since the last time I played with WSPR data mode amateur radio software.
Seeing 10metre band is quite I decided to try out my homebrew 5/8th wave ground plane antenna today with WSPR.
Never ceases to amaze on the results produced. First I set the rig up for about 500mW at the antenna (had to do the coax loss calculations I posted previously). After the first couple of overs the results for my call sign popped up on wsprnet.org spot database. My 1/2 watt transmission was being hear in Moscow 2637km away, also in Germany, Spain, Holland and strong (positive SNR) reception in Denmark and Sheltand all on 10metre band!
Later reducing the power into the antenna to less than 200mw I still had several consistent reports from GM4SLV in the Shetland, and couple in Ireland (Cork) and Denmark.

Trying with QRO (1 whole watt) for awhile to see where I popped up and as I suspected it was not really needed... if there is a path then I would be heard on WSPR with mW's and increasing power did nothing other than get better SNR report from the one path that is being reported... Shetland. This band closure ties in with the other Southern UK WSPR beacons on 10metre. Seems like an excellent propagation tool.

update: Today 11th Jun with 5watts 9076km hop into south Africa...
2011-06-11 10:56 28.126083 IO81vh ZS6BIM KG44df