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DL4YHF > VLF 02.03.99 21:07l 54 Lines 2576 Bytes #-9711 (0) @ EU
BID : 023948DB0EAM
Read: DL9EM DJ9TM GUEST
Subj: Re: TX-LPF for 137kHz
Path: DB0ZKA<DB0ABH<DB0SRS<DB0SIF<DB0NHM<DB0EAM
Sent: 990302/1854z @:DB0EAM.#HES.DEU.EU [Kassel DB8AS] $:023948DB0EAM
de DL4YHF @ DB0EAM.#HES.DEU.EU (Wolfgang)
to VLF @ EU
Hello Lowfers,
Marco (IK1HSS) asked about filters for suppression of harmonics
in 136kHz transmitters. I also experienced problems listening
especially when using a power amplifier running in class C.
Maybe there are other homebrewers interested in this topic,
so I spread the answer here.
The poor harmonic suppression I had in an "old" design
was caused by the high internal induction of the foil capacitors
that i used (taken from the junk box).
An other problem that "standard" foil capacitors often have
high losses at 136kHz and get warm (sometimes even HOT)
at 20 watts TX power.
To get a good harmonic suppression, you should always use capacitors
designed for "high power pulse applications" as in switching mode
supplys.
In germany many electronic distributors (like REICHELT) sell
WIMA FKP1 capacitors. I use the 82nF, 400Volt "FKP1" type in a PI filter
as described below. If you need even better harmonic suppression,
connect more filters in series (not yet tested, but it should work ok).
A good "high power" inductor can be made from Amidon's powdered-iron
toroid cores. I use an Amidon T200-2 (red, 50mm outer diameter).
DL's: Don't pay more than about 5 Euro for this toroid !
For a single-stage PI-Filter at 136kHz with 50 Ohms "input"
and 50 Ohms "output" impedance you need:
2 capacitors 82nF, 1250V DC or 400V AC (more than enough "reserve")
1 toroid coil with 30.5uH, wound on Amidon's T200-2.
The T200-2 has an AL-Value of "12nH/Turn", means you need about
n = sqrt(30.5uH / 12nH) = 50 turns of wire. I use enameled copper
wire with 1.8mm Diameter. Works fine, and turns the ugly waveform
from the mosfet into a nice sine wave (don't know how many dB's that
is, but I will find out soon .... ..). The toroid AND the capacitors
don't get warm, even at 50 Watts.
By the way: I prefer using a broadband transformer to connect
the PA transistor to the output filter. I tried to use an impedance-
matching PI-filter to transform the few ohms from the transistor
output into 50 ohms for the antenna tuner, but that didn't work
good enough and was very critical to adjust.
Using a broadband step-up transformer allows different "taps"
on the secondary coil for different output power levels
(especially for DL's, who want to test their PA with 100 watts
into a dummy load and with 20 watts "on air").
I'm just testing several designs here, maybe more about that later.
Hope to meet you on 137kHz, maybe via crossband or pr-convers !
73's from DL4YHF ("Wolf", often at DF0WD in JO42FD).
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