End Fed Half Wave antenna

[Andyman]

Hi chaps! I am soon to attempt to build a 20m EFHW antenna similar to that described by G0KYA in his blog. My plan was to alter it slightly and use a variable capacitor to take the place of the coax capacitor that he uses in his design. I calculated base on the length of coax he used that I would need a capacitor with the ability to have about 30pf so I ordered an air spaced 8-55pf trimming vari-cap rated for 750V. As I have done a bit more reading whilst the components have arrived through the mail I have become worried that 750V rating may not be enough due to the high impedance of the antenna causing high volatges as I was planning on running 50W through it. I have seen quite a few variable capacitors described as "antenna load capacitors" and these seem to be rated at about 4-6KV.

So really my main question is would I be safe to use my 750V vari-cap or should I buy one rated for a higher voltage in order to run 50W? Im not very experience with electronics so I appologise in advance! Many thanks!

[2E1IIP]

50W should produce around ~225V at 1000 Ohms, so depending on the impedance you should be fine, an end fed antenna can range from 1K to 5K Ohms depending on the design so as a rule of thumb so based on 50W worst case scenario you will be fine with a 750V capacitor leaving you with some head room.
Of course if you want increase the power then you will need something like a vacuum variable capacitor or similar.

[slamjam]

i think you should be alright with 50w

[theEarwigger]

For the capacitor, G0KYA used RG58 coax cut to length.

The person who really developed that aerial was AA5TB. http://www.aa5tb.com

I moved on from it to the vertical end-fed windom which gives you 10m/CB as well as 20m.

-Rob

[Andyman]

Thanks for the replies! Just did a google on the end-fed windom. That is a very attractive/interesting antenna! I presume you are using a fiberglass pole to support it. My intentions were to use a 12m fiberglass pole to support my vertical. Do you know of any good online resources? I have just found M0RZFs website.

[theEarwigger]

Yes fibreglass poles are fine - don't get carbon fibre because it's conductive.

eBay has a number of sellers, I used Paul's Angling supplies for a 10m one. The end-fed windom is more complicated than the LC tuned half wave wire. Getting the transformers right is not easy but the payoff is an extra band, and basically no difference in performance on 20m. On 10m it performs really well because the wire is further off the lossy ground.

Good luck,
Rob.