Blasted RFM69CW

My radio module stopped working last night.
I’d set the output power to +13dB in the new 3 phase sketch.

I’m interested to know if having a longer antenna, full wave length for example, would limit the amount of power being reflected into the chip.

Working into a mismatched load with the output set that high, the RFM69 module is easily damaged.

Quite the opposite actually.
An end-fed full wave antenna exhibits a high impedance, as does an end-fed half-wave antenna.
The RFM69 is designed to work into a 50 Ohm load, thus both would cause an increase in reflected power.

The solution for a monopole is an end-fed quarter wave antenna with a ground plane.

More info, and how-to, here: Improving RF signal strength emonTH emonTX - #16 by Paul

I read that a full wave dipole has about half the impedance of a half wave dipole, thus I assumed an end fed one would follow the same rule…

That piece of foil did the trick, must somehow be reflecting the wave, I guess doubling the antenna length in a way? My Tx was sitting on a metal table, which might be doing the same thing.
So a ground plane doesn’t need connecting to ground?

RF seems fairly mysterious, like some kind of voodoo science.

I didn’t have time to test many antenna possibilities, needing to focus on an install as one does, but I did have time to check if the antenna length at the receiver made a difference.

The receiver antenna on the RFM69Pi. I had a spare module so was soldering different antenna while logging the RSSI from the other.
I had single core 0.5mm wire and tried both 164.5mm and 329mm lengths.
The 329mm performed much better, and to check, I snipped it down to 164.5mm while running… the number of packets dropped meant it was unusable, so I re-soldered a 329mm antenna. I wouldn’t take this test as definitive in the slightest as different modules could have different properties, and there’s a sample size of 2…

I then put on a 329mm antenna on the Tx for what I thought at that point was ‘for good measure’.

That’s for a center-fed (or possibly off-center fed) full wave dipole. Feedpoint location makes a big difference in feedpoint impedance.

It doesn’t reflect, the RF energy, rather it forms one element of a half-wave dipole.
That is to say, it behaves like a one-quarter wavelength element.

In a manner of speaking, yes. It makes a quarter-wave monopole behave like a center-fed half-wave dipole.

Yes. The metal table is capacitively and inductively coupled to the RF module enough to act as
a ground plane.

Depends on the frequency. (among a few other things) At 433 MHz, the ground plane needs to have an RF connection to the feedline. At a lower frequency, say 14 MHz, the groundplane needs the feedline connection, but operation is improved if the ground plane is actually at earth potential.
Note that a DC connection and an RF connection are different animals. Especially at 433 Mhz.

Part of that perception is because we’re used to the way electrical and electronic components
behave at DC.

AC is another story…

As the operating frequency increases, the “rules,” and hence component behavior, change. Further complicating matters is the fact the “rules” change in what can appear to be a counter-intuitive manner.
As if that weren’t enough, the game starts changing when the frequency gets above ~1 Ghz.
Above 2 GHz, it’s a different game altogether.

Bear in mind that’s a very generalized description.

I noticed my error above…

I wouldn’t take this test as definitive in the slightest as different modules could have different properties, and there’s a sample size of 2…

Useful terminology here… end-fed and center-fed.

Does it make sense that an end-fed half-wave worked better for me as a receiver than a quarter-wave?

Importantly, I think I’d already set up the Tx with an end-fed half-wave, so perhaps there was some kind of resonance occurring between the two? And if they were both snipped down to end-fed quarter-wave at the same time perhaps they would maintain their resonance? I don’t know if the physical likeness of the transmitter/receiver antenna shape and size help. Perhaps something to play with…

I saw also regarding dipole antenna than they’re often trimmed slightly short to reduce inductance? The trim length is proportional to wire width and frequency. From what I saw, it was implied that millimeters could make the difference between high inductance or not… Once again, I don’t know how this holds up in practice :slight_smile:

TxShield in plastic case:

The original RFM69CW was blasted using a quarter-wave length antenna at +13dB. Not the standard white cable from megni though, some thinner wire I had kicking around, not ideal either.
This image shows the replaced RFM69CW with half-wave end-fed… and at JeeLib standard power setting 0x99 (in dB I don’t know what it is) I do feel a little heat from the location of the RFM module.

It does. That was determined about 4 years ago during testing of various antennas to determine which one performed the best. It also says the RFM69 module has a decent matching network, as that antenna impedance is definitely not 50 Ohms.

Although the two antennas would be resonant with each other because they’re cut to the same length, the principle at work here is reciprocity. In simple terms, that means what works for a transmitting antenna also works for a receiving antenna.

They would be in resonance with each other, since they would be the same length.
But what we want, is for them to be resonant at the operating frequency.

Indeed it does. e.g. if you shorten the element by bending, or coiling, it will still work, but with
reduced performance. Changing the element length away from resonance changes the antenna’s
impedance. Change it too much, and the transmitter can be damaged.

Here’s an analogy. Think of what would happen if you drove your car at motorway speed and without taking your foot off the throttle, you put the transmission in neutral. By removing the load from the engine, its speed would increase significantly. If you kept your foot on the throttle, and did not put the transmission back in gear, it’s likely the engine could eventually be damaged.

Not to reduce inductance, but due to the property called velocity factor.
In free space, RF energy travels at the speed of light. But in a conductor, its speed is reduced.
Because of that, the resonant length of an antenna element is less than its free-space length.

Increasing the diameter of the wire increases the antenna’s resonant bandwidth. That means it will
exhibit a usable impedance over a wider range of frequencies.
Antenna length, and therefore trim length, is directly proportional to operating frequency.

Not inductance, but impedance i.e. the load the antenna represents to the transmitter and
feedline. (with an RF12/RF69 module, feedline length is nil)
The higher the frequency, the more exact one must be with getting measurements correct.
An error in measurement at 433 Mhz is effectively doubled at 868 Mhz, and a bit more than double at 915 Mhz

You’ll get the best performance from your antenna if it’s as straight as possible.
When I was learning electronics to get my first Amateur Radio license, the gent who taught me showed me a way to make wire stay straight. Clamp one end in a vise, then pull on the free end (pliers, wrapped around a wooden dowel or drill bit, etc) till you feel the wire stretch a little. When you release the tension on the wire, it’ll be very straight and stay that way.

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