Importance of Drain Inductor - L15

[DL2MAN]

Hey guys,

Yesterday, I was publishing my Video about chinese Commercial Radio Batch 2.

While handling the fact that the chinese provided a thicker wire, I remembered an email, I sent to a friend some time ago, where I would write up my experiments with drain inductor coil.

Especially in that context, I found it too important, to not publish my findings, so that maybe others can learn from it, or even proof me wrong. No further delay, here it starts:

OK, what has happened ? All the time we talked about the drain inductor, that was considered uncritical. In most articles this was confirmed, and repeated.
The Spread-Sheets only calculated a minimum Value, and that´s it.
When trying to figure out why some bands in my latest 8-Band built have lower efficiency then others, I was using a thermal camera.
The idea was: When we lose 15-25% of Energy, something needs to heat up.

Basically I never saw loss in a capacitor (exept for the QRP Labs defective ones in QCX times).
Then there´s the FET - which is the only part that I accept to have heat loss, because it´s not an ideal switch.
And of course certain inductors heat up more then others on certain bands. That was the clou I was looking for to solve my efficiency dilemma.

The interesting finding was, that the Drain Inductor was always heated up, regardless of higher or lower efficiency of the band under observation.

So I Investigated this further:

My minimum Drain Inductance is determined by lowest Band (abt. 30µH for 80m), I´ve used 50µH as allround compromise.
The Mini Rinkern Rechner gives also some interesting Information and insight ...
When I type in some additional Information (like Voltage and frequency) it calculates Flux and Core Loss.
Flux was already out of specs for the core I am using and Core Loss was calculated 0,13Watts on 80m (Less on higher frequencies) !
So I played a little bit around, and ended up trying 200µH. This would only mean 0,03W Core Loss.

Theory is theory, and we wouldn´t be us if we didn´t do our homework

So I measured each and every Band (in the still not optimized state) for 3 different drain inductors:
50µH = 13 Turns on FT37-43 (this one I´m also using in my 5-Band Board, and this is the published Choke)
47µH Commercial Coil
200µH = 24 Turns on FT37-43

I calculated Output Power and Efficiency for each Band and Coil.

Without further blablabla, here´s the results:

DIAG1.png (12.04 KiB) Viewed 1501 times

When you look at the first diagram, you´ll see, that in most cases

50µH produces highest output Power. That´s because it has the lowest ohmic resistance followed by 200µH, which has still low ohmic resistance vs 47µH Commercial coil. Latter uses smaller wire gauge compared to my 0,4mm Diameter and is basically reducing the Drain Voltage that can be transformed into RF Power.

DIAG2.png (12.73 KiB) Viewed 1501 times

When we look at the second Diagram (Efficiency vs Inductor) we have a different picture:

In most cases, 200µH produces the highest efficiency (especially on the higher bands), in some cases the 47µH commercial coil does (lower bands), but both are better then 50µH hand wound.
This is because of core loss..... This is with measurement errors involved (my scope is jumping around for Vpp measurements), so I assume the truth is even clearer.

But bottom line is:
The Drain Inductor is not that uncritical we always thought. It´s resistance is affecting output power, and it´s Core Loss is affecting efficiency.

That´s the end of the relevant Part....

Enjoy your Experiments !

73 Manuel; DL2MAN

[John]

Thank you Manuel. The L15 core material, size and wire size are what I am most interested in experimenting with to maximize the class E efficiency. After a lot of reading about core materials I figure they are the easiest item for a manufacturer to screw up, or at least change in a manner that may be detrimental in our circuits. The manufacturers even warn us that there will be differences between batches. Coincidentally they are also the most difficult devices for amateurs to properly characterize.

I do have a question about the efficiency problem though. Has anyone taken a known good (high efficiency,) RF board and started substituting in parts from one of the low efficiency boards 1 part at a time, then testing it? They would only need to do it with the parts that are common to all the bands, and the parts in a single inefficient band. That's a total of only about 10 components plus the FETS. So worse case all it would take is someone with about 4 free hours maximum, 1 of each kind of RF board, and a soldering iron.

[DL2MAN]

Has anyone taken a known good (high efficiency,) RF board and started substituting in parts from one of the low efficiency boards 1 part at a time, then testing it? They would only need to do it with the parts that are common to all the bands, and the parts in a single inefficient band. That's a total of only about 10 components plus the FETS. So worse case all it would take is someone with about 4 free hours maximum, 1 of each kind of RF board, and a soldering iron.

Yes, I did. I exchanged every capacitor, the Fets, The SMA COnnector, The 37-2 Toroids, even the Relay.
The Only thing that I did not change are the SWR Toroids and the L15.... I ordered some (including some with different material) and will continue with my tests as soon as they arrive and I find the time and motivation to continue.

73 Manuel; DL2MAN