Antenna Conundrum...

[Funky Diver]

OK, so I'm generally good with electronics, but one thing has always escaped me.
Transmission doofering....

So, you have your transmitter in your hand.... and you are using an antenna to transmit gubbins to your plane.

We're all familiar with the donut of signal... but at what point does the transmission begin?

The reason I'm asking... do you need to use shielded cable (50ohm if I remember) to get it to the antenna?  Say I wanted to us an array that wasn't stuck on the top of my radio... what's the score.

Apologies for the randomness of my question, if indeed you can get my point, but it's been bugging me... and if you need clarity, I will do a piccie tomorrow to explain (thousand words and all that)

[Funky Diver]

Piccie as promised.



So is the green part's specifications (the patch lead) necessary for any calculations in the antenna length (the red bit)?

[Coyote]

What frequency are we talking here ?

The ideal height depends on frequence, the antenna length for 3db transmission is 1/4 a wavelength.

The cable normally used is 50 ohms yes, but the higher up the frequency range the more db is lost along its length. Say on normal 50 ohm coax you`l get a 0.6+ db loss per m, so with a 5m cable that 3db loss, so really drops your performance instead of increasing it over that length.

[Funky Diver]

Pretty much all frequencies I suppose.

I haven't finalised on my Tx / Rx & VTx / VRx combo yet.
I just used arbitrary figures for now, familiar with attenuation over Co-Ax, I use enough of the stuff, although it's CT63 @ 75Ohm predominantly - as I have bucket loads in the van (Specialist Engineer for Sky, lol)  But being in the receiving game rather than transmission... the answer elludes me, haha.

So long as I'm using shielded coax... and take into account for the attenuation, I'm guessing the tuning of the antenna comes at the point that it emerges at the junction of the tripod / base?

[Coyote]

Yes thats where your wavelength begins from.

[Funky Diver]

Cheers Ian,

I thought as much but for some reason had a brain fuzz that needed smooting out :D

[FPV Matt]

Aye, only active once it leaves the shield. They sell these for 2.4GHz receivers:

http://www.probuild-uk.com/index.php?route=product/product&path=46_85_212&product_id=771
...for CF airframes ;)

I assume much longer at that frequency and it would Be too lossy. Perhaps 35MHz might get away with longer coax. Sounds like time for an experiment!

[Funky Diver]

Cheers Matt,

In all fairness I'd probably make my own (being handy with the tools required), but great for the link, ta.