27Mhz - Radio Ideas

Hi David,


https://en.wikipedia.org/wiki/Antenna_tuner

Regards
Jonathan

I typed a response here yesterday, I detailed that is was not well thought out and I might regret content.

I deleted the post as some of what I typed did not make sense and made me doubt my understanding of the basic principles of antennas.

So I pondered - did I ever know this stuff (yes I think so), have I forgotten what I knew (don't think so) - am I certain in my understanding - No.....

Net result is I thought I would delve into the world of VNA's and see if practical experiment(s) can shore up the connections in my brain

I used to have a very capable antenna analyser made by MFJ £500+ but sold it for peanuts along with the addition of free dust.

So whilst I can see a steep learning curve ahead - I am looking forward to loosing more brain cells...

Regards
Jonathan

Shame you sold it!

Rob from Holland writes to recommend an "MR300 antenna analyzer which should contain an AD9851 chip."

I wonder if one used (say) a CB transmitter on low power to try to transmit to the underwater antenna that an ordinary SWR meter would work??

But yes antennas are a bit of a "black art" and I am only at Foundation level!

David

M7DYP

That was mighty quick service - delivered in less than 24 hours from point of ordering...

Regards
Jonathan

david f wrote:Shame you sold it!


I wonder if one used (say) a CB transmitter on low power to try to transmit to the underwater antenna that an ordinary SWR meter would work??

Might fire up my FT736R 10M | 2M | 70cms | 23cms and use the very capable 10M (11m) module for experimenting...

Regards
Jonathan

Very nice!




I am also impressed that you have gone for a VNA.


Good luck and don't forget to try an underwater dipole! (About 535 mm overall length by all accounts.)


David

I just came across a very interesting video regarding experiments using the 27mhz toys, we always knew they could be made to would work if we could get the pulsing to stop and transmit continuously, it looks like someone figured how to do it on one of the 27mhz toys, the chips might still be useful for a low cost version even with the current code. I even see a regular 27mhz crystal in there!. From what I can tell we should be able to use the transmitter modifications pretty much directly for a ppm test, the only thing is I'm not sure how we can modify a receiver chip to get a raw signal back out but at 5x pairs of chips for around £2.50 delivered it's very cheap and worth looking at.


https://www.youtube.com/watch?v=mijxbYfPCq0

https://upverter.com/datasheet/97685b43476ad62a2450315e7f924625c8c8f6c7.pdf

https://www.youtube.com/playlist?list=PLA5a2xPRSrB1mLQvgf5ILMeTcRMuDtcNa

https://www.aliexpress.com/item/1005002505371749.html

Is this example using same chip TX2?

https://github.com/carbofos/avr-pc-radio-control-27mhz

Regards
Jonathan

C-3PO wrote:Is this example using same chip TX2?

https://github.com/carbofos/avr-pc-radio-control-27mhz

Regards
Jonathan

that looks like the same chips he is referencing in the russian post,

TX-2 chip output pin is 8. So you can remove buttons and TX-2 chip or leave it and use your transmitter board as is by connecting the AVR output pin to the pin 8 of TX-2. Do not forget to connect VCC+5V and GND to your transmitter board.

thats makes a very simple 27mhz transmitter. but im not sure if there is a raw output pin on the RX chip.

https://skootsone.yolasite.com/toy-rc1.php

https://skootsone-yolasite-com.translate.goog/toy-rc1.php?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en-GB



found a few other pages about those chips.

https://www.instructables.com/Arduino-controls-cheap-RC-car-transmitter/

https://forum.arduino.cc/t/rc-car-project-help/355557/15#msg2544112

The NARSA (Northern Amateur Radio Societies Association) Rally at Blackpool  had some useful stuff at reasonable prices.

I  invested in  a NanoVNA - H4  network analyser (About £80 from Mirfield Electronics) to hopefully do some underwater antenna development. (The spec says it can measure up to 900 Mhz)



The book from the RSGB (Radio Society of Great Britain looks useful too.

im currently shopping for a sub and went looking to get the radiometrix modules but i cant find where sells them? what site did you order them from?

Well, I was about to say don't bother and save your money until yesterday when I tried my E11 sub fitted with Radiometrix 27Mhz modules with the latest version of the underwater antenna. It actually worked quite well. So it shows promise, I would say.

This photo shows the underwater dipole with an overall length of 534 mm. The foam, in the middle,  covers a 1:1 Choke Balun on a ferrite ring. I have used foam to try and avoid putting the Balun in a bulky, waterproof box. A blue LED at the stern is triggered by a "surface planes" command.

I am very grateful to Rob  Schuckman for pointing me in the right direction:

https://www.robschuckman.nl/

Rob recommends using a trimmer capacitor. But as a very rough initial trial I put a 22pF capacitor in one leg of the dipole and this seemed to improve matters (range, glitching etc). For approaches to a more precise approach see the end of this post.





This gives an idea of the range (on the surface but with antenna immersed.)



This photo perhaps shows the direction of future development - the use of a nanoVNA to optimise the underwater antenna:

(I managed to measure and store "S Parameters" for this antenna in air and water. If anyone would like to have these (and hopefully understand what to do with them more than I do!) then please PM me.



Various video clips showing yesterdays trial. (The trials were untethered but without diving. Small steps! ) :

https://youtu.be/bvnZf2CLhzQ

https://www.youtube.com/shorts/fpwXxoxftMY

https://youtu.be/Gy7s8oT-c_M

https://youtu.be/IjNQdDVIzcU


My Arduino software version I am using is here:

https://github.com/rdforrest/27MHz--Radiometrix-module

Radiometrix-RX-3.8 and Radiometrix-TX-3.2 are the current versions.

Sorry Geofrancis, I forgot to give the ordering information for the modules. I phoned:

Jim Pareathumby
Sales & Admin Department
Radiometrix Ltd
Hartcran House
231 Kenton lane
Harrow
Middlesex
HA3 8RP
England.

Tel: +44 (0) 20 8909 9595

The modules are not cheap and I wonder if the project could be done more economically by purchasing a TX module (which gives the advantage of CE approval) and using it with a cheaper RX ?

what the S parameters could allow you to do is design a matching unit; assuming the Tx/Rx are optimised for the 'air' then the Tx/Rx impedance should be almost the same as the 'air' to maximise the power transmission.
The ATU then matches 'water' to the 'air' again to maximise power transmission.
If you want some real brain freeze look up 'smith chart' its a way of displaying this information and more usefully, design matching pads graphically rather than maths formulae.
For adjusting I suspect an SWR meter is better as it directly measures forward/reverse power flows, but if you measure the pad+water antenna with the VNA the aim would be to get it as close to the 'air' values as possible.
Mind you, that would mean getting the VNA close to the water!

I had to laugh at this one.

We had the Furness MBC Regatta this Sunday. I tried the 27Mhz testbed (My E11 sub) again.

Paul Brassington was a guest and we were talking together whilst sailing and it turned out that he was also on 27MHz , fortunately on different frequencies! (The Club pegboard was thrown away years ago but the fact that we both had great big aerials should have been a clue.)

Paul's home made (in the days when you could) TX on 27Mhz:



A video of us both sailing on 27Mhz (Sorry the video is occasionally out of focus - those blummin big aerials again, upsetting the autofocus) :

https://youtu.be/TJ2OaX-ICBo

So 27Mhz is not as defunct as we may think?

A postscript to this.
Paul has just sent me copies of the original article describing his TX & RX. (Radio Control Models & Electronics July 1979) It makes for interesting reading. The TX is FM, 8 channel and transmits at 600mW with a crystal. The encoding system uses an NE555 as a pulse generator followed by an MC14017 decade counter. The RX uses a crystal and an MC3357P as a radio chip and an MC14015 as a decoder. There are some hand wound coils but I suspect that many components e.g the IF transformers would be hard to obtain now.
Important information is that the aerial is just 9 inches long - not sure if it is inside the WTC or in water. (I have sent this question.)
Later edit. Paul confirms that he uses a vertical aerial insulated from the water, 9 inches long from RX to tip.

Well I have just been trying this 9 inch simple aerial in my small test tank with the nanoVNA with promising results. An SWR of about 5 at 28Mhz.

Some caveats:

- It is only a small test tank not a large body of water.
- I am still getting used to using this nanoVNA and I may be  guilty of misinterpreting it completely!

But:
- A 9 inch aerial in water forms a very practical antenna for our use. It could simply be fed forward out of the WTC to bow or stern or periscope (Like Paul's). The 0.5 metre dipole with Balun was awkward to use on models. (See my photos)
- It has to be worth a trial when next I go to the Lake.









Later edit:

For anyone wanting to find out more about using nanoVNAs i found this YouTube video very helpful (among lots of poor videos on the topic) by Andreas Spiess. (NanoVNAs are complex devices and seem to come without any form of manual!):

https://www.youtube.com/watch?v=91ZRTFZ40rw

Well that was a failure!

The photo shows the maximum range.

I ended up having to  be rescued. Thanks Guys! Mind you the sight of a Russian sub breaking down lifted my heart. Slava Ukraini !




This one shows the 12 inch aerial enclosed in the 4mm OD nylon tube. (Useful stuff):




It is a shame  because the 12 inch antenna would be very practical for model subs compared with the dipole, which did give usable range.

458Mhz was sooooOO much easier to get workable results with!

Maybe Paul's better range was down to 600mW of TX output?

I think I will get back Rob in Holland and see if he provide a COMPACT dipole with antennae matching circuit.

Later Edit for the Techies out there:

A Smith chart and SWR measurement made on a 12 inch antenna immersed in the lake (i.e a large body of water as opposed to me test tank.):



This is showing resonance at 90MHz so maybe 12 inches is too long?

can you slow down the transmission any more?

what about a test program that just sends some fixed data repeatedly but slower each time. I wonder if there is a magic number out there that it will start behaving like the analogue system.

Thanks for the suggestion.
However I think the problem now is getting a working and practical antenna for model subs. (The software isn't bad - in my opinion anyway!)
I think I will roll-back to the dipole with balun and a series capacitor. That fits within Rob's suggestion and worked quite well.
I intend to give it another test in a sub on Sunday.