Aerial compatibility question...

[Dodgy Geezer]

This is somewhat off-topic for vintage R/C - but I think it requires the skills that only a vintage r/c specialist would have. It certainly doesn't get any responses on RCGroups, so I'm casting my net wider....

A little while ago I dropped a 2.4Ghz Radiolink T7F down two flights of stairs onto a hard surface (as you do ). They are solid beasts, and the only thing that broke was the 2.4Ghz aerial at the hinge. It works OK with duct tape holding it on, but I would like a (cheap) proper repair. And it has an unusual push-fit into the case, so It would be hard to convert it into a screw-in.

Now, no spare aerials for the T7F are advertised. But there IS a cheap one for the Radiolink A10 - with the right push-fit. https://www.ebay.co.uk/itm/Radiolink-Re ... SwQiFaYFwi

It's different inside - it has a PCB aerial rather than the wire one of the original. (The one I got is different to the ebay picture) But its physical fit is perfect, including the cable connector.

My question is - Is there any reason why this might not be compatible?

For 27Mhz sets you need to tune the aerial to the right length, and even if you got a replacement that was nominally the same it would make sense to check the balance before using it. But as far as I can see, all 2.4Ghz sets use the same antennae, and so an aerial for 2.4Ghz ought to do any service - even wi-fi. Unless, of course, someone here knows different...?

[PaulJ]

First I must say that I am not "electronics literate" so you can't take what I say as gospel, however for what it's worth......... When I convert old transmitters using Phil's various emulators, I like to keep the tranny looking as much like the original as possible which means leaving the original telescopic antenna in place and hiding the 2.4 GHz antenna inside a dummy frequency pannant....... and that means making my own antennas.

('Don't know why the picture came out on it's side but you get the idea)


So how long should the exposed bit at the end be? Applying the formula for a quarter wave came out at 28mm so I made them that length and they worked fine with good range. However I have subsequently dissected an "official" antenna as supplied with Orange Rx transmitter modules and they are only 26mm...... In both cases it is "mirrored" by a bit of brass tube soldered to the connector screen to make a dipole antenna. So my last three home brewed antennas have been 26mm and they seem to work just as well with excellent range

So the long and the short of it (excuse the pun!) is that, within a mm or two, I don't think it is that critical and I would just try it and see if it works....... I think the currents concerned are so low that you are unlikely to burn anything out but perhaps someone more qualified would confirm?

[Spike S]

Also, see Phil's post elsewhere on this Forum:
viewtopic.php?f=62&t=110

[Martin]

I bought a (relatively) cheap Antenna analyzer - N1201SA - from AliExpress after watching this video. It's not really cheap - about a hundred and fifty pounds when you also buy the calibration plugs - but much cheaper than the professional grade VNAs that cost thousands.



I tested all my detachable 2.4 GHz antennas and found quite a wide range of frequencies they were best tuned for, ranging from about 2.1 to 2.6 GHz - but they all work fine and have pretty good SWRs at the 2.45 GHz point.

What was more interesting was testing various antennae that I've received for various pieces of kit over the years that run at 433 MHz, 5.8 GHz and the GPS frequencies of 1.2 and 1.6 GHz. Many of these are just 2.4 GHz antennas - I even have one that is labelled 433 MHz, but is really a 2.4 GHz and quite useless at 433 MHz!

Unfortunately my VNA only goes up to frequencies of about 2.7 GHz so is no use for checking / optimizing the antennas I use in the 5.8 GHz range for video FPV - but it has identified that quite a few of the supplied 'rubber duck' antennas that I've received with cheap FPV gear are actually 2.4 GHz ones!

It seems that some Chinese sellers of equipment that require antennas just supply the cheapest antenna they can obtain with the right connector - these are usually 2.4 GHz ones as those are the most common.

[PaulJ]

Also, see Phil's post elsewhere on this Forum:
viewtopic.php?f=62&t=110

'Just read this post and I notice that the length of the exposed bit at the end (26mm) is different to the length of the brass tube "other half" which is given as 24mm....... The antennas I have made have all had equal lengths, as did the the one that came with the Orange rx Transmitter module(26mm). My antennas seem to work OK so what is the theory behind having slightly different lengths?

Also, the first few antennas I made were all fairly short and were made from the co-ax connector that was supplied with the Tx module and they all work well with no range issues. Then for one of my transmitters I needed a longer peice of co-ax and sent off for a longer connector off ebay. When it came it was of a thicker guage co-ax than I had used up to that point but in line with my "try it and see" philosophy, I did make an antenna from it and it too seems to work fine.

As you can see, the "Mk2" antenna is considerably thicker than the original ones.

I have now read something about the importance of matching the impedance of the connector to the antenna so have I just been lucky and got away with it or was this potentially a serious mistake? Perhaps I need to fork out for one of Martin's analysers

[Martin]

Paul, I wouldn't bother to buy one of those VNAs unless you're particularly interested in the subject or want to try designing some non-standard antenna of your own.

For the antennae you're making for your 2.4 GHz sets, I think the design you're already using is perfectly good. I guess the different lengths of the tube versus the whisker are to do with velocity factors - but I'm not sure.

I don't think the dimensions are super-critical. Obviously you should try to be as accurate as possible, but if you're a millimetre or two out the aerial will still work fine.

It is important to realize that all the coax cables we use (all relatively light and thin) are all quite lossy at 2.4 GHz, so you shouldn't make the non-transmitting (intact cable) part any longer than it needs to be. If you have a foot or two of coax feeding the active part of the antenna (the modified bit at the end) then you can easily lose half of the available signal.

[PaulJ]

Martin, many thanks for that....... Does the guage/thickness of the feeder make any difference, would a thicker cable be more or less lossy than the thin one?

[Martin]

Generally speaking the less lossy cables are thicker, more rigid, and much more costly. As the frequency goes up, more power tends to be lost and at 2.4 GHz a low-loss cable to run an appreciable distance would be much too large, stiff, and heavy for use in our models, or even transmitters.

Generally we only need the cables to be a few inches long: at those lengths there's not much loss even in the really thin cables. The choice of which type to use generally comes down to more practical considerations such as how easy it is to solder, or otherwise connect, it to whatever sort of connector is provided by the transmitter or receiver you're connecting it to - or with the antennas used on racing quads, the cable needs to be strong and stiff enough to mechanically hold the transmitting part steady without it whipping around in the airflow.

[jackdaw]

If I remember my 'ham radio' info from when I took the exam back in 1984:

1/The composition of the insulation on the active element affects the velocity ratio and therefore the length.

2/The ratio between the length and diameter of the conductors also affects the velocity ratio(tube bigger dia thus shorter).

3/Feed impedance can be altered by adjustment of the passive element and in the case of multiple passive elements by angling them at less than 180deg relative to the active element. Nominal feed impedance of a half wave dipole is 75 ohms. But, with say 4 passive elements you can 'splay' them out and bring the feed impedance to 50 ohms.

Google it for a fuller explanation.

ps. Hope my memory is ok!

[Martin]

Perhaps it's time to move this discussion to a new thread rather than continue this one in the 'Help' section. Admins, please feel free to do that if you think it's appropriate.

Paul sent me some antennae to test with my VNA. I also made one of my own, with the 'whisker' intentionally longer than the reference design, as I was intending to snip bits off the end, a little at a time, till it resonated in the middle of our band.

But my tests didn't give the results I expected: some of the antennae had a strange frequency response with two different resonant frequencies; in general the antennae were found to resonate at higher frequencies than I expected. it seems that we might need to make them a few millimetres longer than we thought, if we want them to be optimized to work in our band.

Anyway, here is my video. Sorry it's a bit long and rambling. Maybe some of the experts here can provide some theories as to why I got the results I did.