RF meter

[Burgerman]

I made a field strength meter. But it doesent work right...

My electronics is crap!

It works perfectly at 900mhz. At 2.4ghz it reads almost nothing.

It consists of some super basic components.
1. an antenna (dipole) for 900 or 2.4, rp-sma swappable.
2. a bridge rectifier with 4x of 1n5711 diodes.
3. A 50 microamps panel meter...
4. in a small plastic box.

I suspect the diodes that I used dont like 2400mhz...
Because it works great on the 900 band... As I was expecting. Even several meters away.

What diodes can I use? Is that even the problem?

[Kande_ian]

My electonics is crap as well and this topic is touching on thing I haven't worked with in nearly 40 years but my first thought is that 2.4GHz takes you into the region where component size and layout matter because they are not insignificant relative to the wavelength. Is there anything else at all in your circuit and how is it wired up?

[Burgerman]

I used a 900MHz (0.9GHz) or a 2.4GHz dipole "rubber duck" type plastic antenna
On a RP-SMA mount with short coax to a soldered small simple bridge rectifier with no non coax cable used. Components soldered to the diamond layout 4x diodes directly. With the 2 DC output wires from the bridge directly to a 50 micro amp cheap chinesium meter.

On 900 I get full needle deflection (50uA) at 1 meter (500mW Tx RC system output). It is antenna polarity dependent. As in antennas must be parallel of course.
0n 2.4GHz I get full needle deflection too, but only if range is zero! Touching antennas... At 2 inches? Nothing...

My brain cell tells me it should work... Reality seem to doesent agree.

Been considering Agilent HSMS-282x Surface Mount RF Schottky Barrier Diodes in place of the ones I used.

What do you think?

[Kande_ian]

My knowledge of RC stuff is non existent - past experience was with lab equipment based on repurposed 1940-50s era UHF airborne radar sets with disc-seal triodes and coaxial resonant cavities which were closer to plumbing than electronics.
With the scenario you describe my starting point would be to want to have separate verification that there is actually some significant power being transmitted at 2.4GHz, could it be that there isn't anything much to measure and your detector is telling you the truth?

[Burgerman]

The 900 and the 2.4ghz are both configurable from 10mW to 1W output. I have both set to 500mW. Both light up a NEON "bulb" taped to the antenna equally.
Both have similar range, both use the same LoRa protocol. So both output the exact same rf signal just at different frequencies.

So both should have the same exact signal strength (at close range at least). Over a longer distance (miles) the 900 is absorbed less by the surroundings or water in the atmosphere (humidity levels). So the 900 gives both longer range and slightly higher latency as lower frequencies carry less data all else equal.

[Kande_ian]

So you have your antenna attached to soething like one of these https://uk.rs-online.com/web/p/coaxial-connectors/1810013? (I've just discovered I don't know how to embed an image in a message).
How about dispensing with the co-ax and soldering your bridge directly onto the back of the connector - that takes cable matching out of the picture?, otherwise is your coax around half a wavelength at 2.4GHz - (about 4cm in coax? ) or integer multiple? As I said earlier, at these frequencies small changes in dimensions can become significant.
If still nothing - I'd probably look at trying to use a single diode as a basic diode detector or connecting bridge directly to Tx output through an attenuator to try and get a handle on its sensitivity.
Have you any means ofvadjusting or sweeping the TX frequency or are you limited to fixed bands?
All of the above to be treated with scepticism because I'm reaching a long way back in my memory.

Ian

[Burgerman]

So you have your antenna attached to soething like one of these https://uk.rs-online.com/web/p/coaxial- ... rs/1810013? (I've just discovered I don't know how to embed an image in a message).

Yes.
Except it includes the already fitted correct length coax with a tiny apex 1 connector on the other end. I use these on RC cars/planes etc all the time.

How about dispensing with the co-ax and soldering your bridge directly onto the back of the connector - that takes cable matching out of the picture?, otherwise is your coax around half a wavelength at 2.4GHz - (about 4cm in coax? ) or integer multiple?

Yep...
Except that theres actually 2 antennas, and two cables, and 2 bridge rectifiers, and while both were intended for 2.4ghz recievers, the one thats the wrong length for 900 works great!
I think with dopole antennas it makes little difference how long the feed cable is. But its correct for 2.4...

As I said earlier, at these frequencies small changes in dimensions can become significant.
If still nothing - I'd probably look at trying to use a single diode as a basic diode detector or connecting bridge directly to Tx output through an attenuator to try and get a handle on its sensitivity.
Have you any means ofvadjusting or sweeping the TX frequency or are you limited to fixed bands?
All of the above to be treated with scepticism because I'm reaching a long way back in my memory.

It hops on the 2.4 band across 23 channels, from 2.4 to 2.483GHz and the 900 is from 902 to 928MHz. No way to change or sweep. Just turn on!

For what its worth a single diode does the same thing, but is less sensitive on both frequencies.

I am presuming the switching speed of the diodes isnt really fast enough and so is hindering the detection of high frequencies like 2.4. So I ordered some,

[Kande_ian]

That looks like it should be a very neat package (though these are hard to beat for sheer beauty https://www.r-type.org/exhib/aaa0024.htm and they make great paperweights).

I misread your original post and assumed "rubber duck" referred to the quarter wave end fed type antenna not dipoles - not that it matters particularly apart from envisaging the setup properly.

Except that theres actually 2 antennas, and two cables, and 2 bridge rectifiers, and while both were intended for 2.4ghz recievers, the one thats the wrong length for 900 works great!
I think with dopole antennas it makes little difference how long the feed cable is. But its correct for 2.4...

I agree, I don't think the lower frequency will care that much if the feed is short and connected to something.

It will be interesting to hear if the new bridge sorts it - it looks like this site might revive old interests as well as help with my latest (I just heard from shirley this morning that the new adapters and wheels for my wife's Sango have arrived in the UK - only 8 days since he shipped them. Then there's R-net to understand! I may never find time for work ever again).

[Burgerman]

Thats my problem. Well that and rather a lot of others!

Too much stuff to inverstigate, fix, improve, and try. Right now I am building a long range FPV (first person view) plane. I fly RC like I drink. It just comes naturally to me!
That gets complicated.

So I dont even think about it.

However if you want to be amazed watch this guy fly RC...

He makes me want to sell up and go home.

Just watch from 13 minutes dead.
Here, pre started point! https://youtu.be/KUr0Odt8peI?t=808

And this is him just getting warmed up. No not Cleetus, his freind.

Tell me what you think.