Base loaded antenna

[j8m8l]

Hi Guys

I am interested in making a base loaded antenna like this (http://www.rcgroups.com/forums/showthread.php?s=&threadid=113873&) for my rc planes.

I have found several "designs" for these, but they are all for 72mhz, and over here in the UK we fly 35mhz.

Does anyone know how i can calculate the correct value inductor required to make this work? I have asked my physics teacher but he has no idea :(.

Thanks for your help

Josh

[vintage1]

Sadly, my antenna theory is too rusty.

However, I did find a little program that will calculate it all for you.

IF you know what your RX antenna feed impedance and ground impedance is (assume 4 ohms)

http://www.btinternet.com/~g4fgq.regp/vertload.exe

G'luck m8!

[j8m8l]

Cheers for that !

Not sure what the impedance's are, but i will get some help with it.

I might just try some trial and error stuff, with a 2-3 henry too.

Thanks

Josh

[vintage1]

I'd guess at 2-3uH actually.

You may need to re-tune the input ciol on teh reciever as well.

Frankly, if you just take teh bottom half of the antenna and coil it around sometunng plastic or a bit of fiberglass (NOT carbon fibre) rod or tube, and retune the front end, you are probably getting there.

Then try various lengths and coils till you get the best range.

Each reciever will be slightly different, as the nmanufacturers merely make sure that a reasonable preformance with their bit of wire is the order of the day.

In teh end, what matters is the range check. There may be some degradtion of interference rejection if you end up gerossly mismatched, as well, so be careful.

Its hard to develop this stuff without expensive gear and some knowledge, but you can do a lot by just tweaking teh antenna and input tuned circuit and looking for best range on a collapsed TX antenna. BUT YOU ARE ON YOUR OWN with this.

A bottom loaded antenna that is short will never do the range of a full length antenna, even if perfectly matched.

[j8m8l]

Thanks again

I ment uH (whoops)

I am going to have a play setting up a "full length" with the 2 or 3 and then cutting it down and range testing.

Josh

[j8m8l]

I had a look at RS (http://rswww.com/cgi-bin/bv/browse/Module.jsp?BV_SessionID=@@@@1951967305.1051974749@ @@@&BV_EngineID=cccdadciemgkggicfngcfkmdgkldfhg.0&3229427582=3229427582&catoid=-96501091)

I think that i will get a 2.2uH 3.3uH and 4.7uH

Unfortunatley i will need to get 10 of each, so does anyone wanna split?

Josh

[vintage1]

Why not get 10 of teh 4.7Uh, then unwind the wire on a few and cover with half and 3/4 the turns...to get the other values?

[j8m8l]

I dont think that i am gonna get the wound ones, rather the ones that look like little reisistors

I am going to look elsewheres (rapid) too see if i can get less

Thanks

Josh

[vintage1]

well..they probably ARE wound..then dipped in epoxy.

Maplin (www.maplin.co.uk) do moulded 2.2uH and 4.7 uH chokes at 49p each..

And a range of the sort that you want at a penny each...Hmm. See part numbers

SF52G - 2.2uH
SF54J - 3.3uH
SF55K - 4.7uH

These are wound on ferrite type chokes.

Impedance at 35mHz would be 500-1kohms...
Seems a LITTLE high to me.

However, my theory is rusted beyond redemption :)

Why not get a few, test out the theory, and then go for the smaller and neater ones later on?

If you can identify the chipset used, and work out which is the AGC line, thats about the bets way of detecting 'on tune' when you fiddle with the inpiut coil, and would gice a very rough guide to sensitivity....

My futaba uses something marke TA 750 and 7751F or somesuch..

Let me know how you get on.

[j8m8l]

Thanks

Maplins it is (i dont usually look there 1st)

Josh

[HarryC]

I should point out that use of this proposed aerial will invalidate your transmitter's Type Approval and mean that it is no longer legally approved for use in the UK.

Transmitter manufacturers are required to submit their transmitting devices to the regulating authority and gain a Type Approval from the authority. Without Type Approval a transmitter is not legal for use in the UK. The aerial forms an integral part of the device that is submitted for T.A. Using a different aerial to the type was used in the T.A. submission means that the transmitting device is no longer the one that was submitted for, and gained, T.A. It may work perfectly well. It may gain T.A. if you or the manufacturer submit it, and pay for the T.A test. But until that is done and a T.A. certificate is issued, it is not legal to use it.

I want to use a different, well respected, German made aerial on my Tx, and it is made specifically for my transmitter but not by the original manufacturer. I checked with the BMFA and they confirmed that it definitely does invalidate the Type Approval in the UK for the reasons given above.

H

[steve lewin]

Harry,

Interesting points except that the discussion is about receiver aerials which are not regulated in the same way. Are they ?

Steve

[vintage1]

You beat me to it :)

No one in their right minds is going to mess with the transmitter...well some may, but I certainly would not recommend it.

This is about getting a bit more range out of a cut down receiver aerial.

[j8m8l]

Thanks Guys

I am on study leave soon (darn GCSE's), so in 3-4 weeks i will have some time to play with this.

I am gonna have one LONG summer holiday this year ;)

Josh

[HarryC]

Hah, I should have followed the link in his original post and would have seen he meant the Rx. I assumed he meant the Tx. It is worrying the number of people who do try to radically alter the Tx aerial, and a UK model radio importer is advertising an independent aerial that invalidates the type approval on the Tx that they import!

Harry