Power Attenuator by Mark DL6YYM

Mark DL6YYM designed a power attenuator for 5 watt, using resistors of 1/2 watt. The attenuation of the attenuators, from the left to the right, is 20 dB, 10 dB, 3 dB and 3 dB. Seen from the front. This enables Mark to lower the power from 0 dB to 36 dB, in steps of 3 dB or 4 dB.

Click to enlarge

Mark built a beautiful enclosure for the attenuators.

This picture shows the interior. Seen from the back, you can see the 20 dB section on the right, behind the coax. This section has the larger number of resistors. The 10 dB is visible to the left of the 20 dB section.
At the left you can see the two 3 dB sections, each built with a total of 7 resistors.

Click to enlarge, to see all the details.

Mark used the PA1B Power Attenuator Calculator, to design the attenuators.
You can find the Calculator on the PA1B QRPp page,  This Calculator in Excel enables you to design an attenuator for any power, using common resistors from the E12-series. E.g. 1/2 watt. Mark used Open Office. Please note, that in Excel or Open Office you must use the decimal COMMA or POINT according to your national language. These attenautors are what I had in mind, when I developed the Power Attenuator Calculator. hi

The shack of DL6YYM.

I thank Mark for sending the pictures and for the great feedback on the attenuators.

Attenuators and tuning

50 Ohm attenuators
The attenuators shown on the PA1B Attenuator page are all very accurate 50 ohm attenuators. The accuracy is obtained by placing two, or more,  resistors in parallel, to form the required ohmic values very accurately, with resistors from the E12-series.
But the accuracy of the attenuators is also dependent on the termination of the attenuators. 
When the attenuator is terminated with 50 ohms, the attenuator will show also an input impedance of 50 ohms and most important, will give the right attenuation.

Setup

The attenuator will be placed behind the set. The SWR-meter is not shown in the drawing, but is placed between the attenuator and the tuner (transmatch). I work with this setup, as long as I work with the attenuators and by doing so, I made many fine QRPpp QSO's, using an inverted Vee as antenna.

The right setup.        Tune, as accurate as possible, for a SWR of 1:1,

Tuning

Tuning is done with the attenuator on bypass (0 dB). Tune carefully to get a SWR of exactly 1 : 1. When the SWR is 1:1, the input of the tuner will show an impedance of 50 ohms, so later with the attenuator switched on, the attenuator is terminated with 50 ohms and the attenuator will give the right attenuation. It's very important to tune for 1:1 as accurate as you can, to get a termination with 50 ohms for the attenuator.

SWR

I got an e-mail from a follow Blogger, Mike VE3WDM. He was using the autotuner of the Elecraft K3. The K3 is connected to  the SWR-meter, which is connected to the attenuator, which is connected to coax to the antenna. Mike wrote that he had a SWR of 1.6 : 1, with the attenuator on by-pass. Since the autotuner is in the set and the SWR-meter is behind the set, this means that the coax to Mike's attic antenna shows an impedance of 31.25 ohms or 80 ohms. (50/1.6 or 50x1.6)

The e-mail of Mike made me realize, it is not possible to use the autotuner in the set, together with the attenuators. The tuner has to be behind the attenuator. However, it is always possible to use another (homebrew) tuner behind the attenuators. Mike later said that he will place a tuner behind the attenuators and will turn off the tuner in the set . 

So place the tuner behind the attenuator and tune as accurate as you can to get a SWR of 1 : 1, to get the right attenuation. 

Parasaki

Yesterday I received the schematic of a very remarkable transceiver in an e-mail from Peter DL3PB.
The Parasaki transceiver is an all diode receiver which is built with tunnel diodes in stead of the usual transistors. Peter proudly said, that a long cherished dream came true, after 40 years, to build and operate a transceiver build with tunnel diodes.

But that's not all. After completing his temporary antenna, Peter heard the very, very strong signal of Christophe F8DZY, in the France REF contest. When Peter answered his CQ, he got an immediately correct response, although his power was only 2 milliwatts. hi

Parasaki Transceiver Power 2 milliWatt  Peter  DL3PB

This is what Peter wrote in his first e-mail:

Hi folks,

I'd like to share with you a long-cherished dream, that recently came true, fourty years after I came to read about hams using tunneldiodes to make QSOs when I was aged twelve or so:
Finally I managed a first skywave QSO with my PARASAKI-transceiver, an 'all diode' rig:
Christophe F8DZY replied to my very first call on 20m band in REF-contest last weekend.
I was running 2mW into a temporary vertical dipole on my balcony. Distance between us is 918km - obviously OM Christophe has excellent ears.
Those interested in the cruel details of my circuit, please find attached a schematic and a photo of the pretty ugly setup. The circuit is designed straight-forward with exception of the parametric VXO, derived from Mike/AA1TJ's famous Paraceiver design. (see http://fhs-consulting.com/aa1tj/paraceiver.html )
The low impedance of the high peak-current tunneldiodes make it very difficult to built a really crystal controlled oscillator rather than an LC-oscillator, synchronized by the crystal more or less, at least on the higher SW-bands. The Parametric VXO provides a crystal-stable, chirp-free signal on expense of an output power of two milliwatts only instead of ten, but with an amazing spectral purity, no need for a low pass filter or such.
Of course it sounds pretty cool making a QSO with a 'bunch of diodes' and a parametrically excited crystal, but believe me or not, I'd preferred to bring that full ten milliwatt into the air - on the other hand that approach allowed to tune the rig a bit ( ~ 5kHz/per xtal ), which turned out to be much more valuable than a few milliwatts more while being 'rock-bound'.
The receiver in its 'gain-less' version works fine for strong signals - while listening to QRP(p) stations, the moderate gain of the audio amplifier helps a lot. A comfortable frequency shift between receive and transmit is realized by the 5µH inductor at the LO-port of the mixer, with little effect on sensitivity.
Thanks for the bandwidth, OMs, won't bother again you with such mails, unless I make a cross-pond QSO with that rig ( not that likely ) or any skywave QSO with homemade semiconductors ( probably impossible )...
72! Peter/DL3PB

In my answering e-mail I said:

As a milliwatt enthusiast my self, I know the thrill of making contest QSO's with a few milliwatts. Your 2 mW reminds me of the QSO's, I made in 2001 to 2003 with 2.5 mW to 10 milliwatts in many different contests.
It is amazing what a "simple" circuit like this can do. I like the set up on these experimenting board. It's great fun. I think you made an excellent choice to choose for "broader" tuning range. But "on the edge" a few dB can really be helpfull. hi
Yes, Christophe must have excellent ears. But with so little amplification in your receiver, you also must have excellent ears.


The next day Peter replayed with:

Hello Bert,
when I actually had set up my antenna last saturday and while adjusting my rig,
I heard F8DZY call CQ test - he was really booming in, and I recalled your strategy, to monitor the signal strength of the incoming signal in order to find the best time for a call.
I have no S-meter, but he was peaking such, that I was sure that couldn't last for long - so decided I'd better give him a call immediately rather than trying to squeeze the last tenth of milliwatt out of my rig - so I did and before I could repeat my call once more, he came back with my callsign and the department number - so it payed off to study your miliwatting results carefully!
Sensitivity of the receiver is not that bad, given the low part count - with the audio amp and a sound powered' headphone I can copy EU-QRP stations pretty good. Of course that won't work with a TV playing in the background, but I'm blessed with a wife, who is willing to use headphones on occasion.
Thanks a lot, Bert, please keep up your great blog!
Peter/DL3PB

Congrats Peter, on your great achievement!

Click here to go to Roger G3XBM's Amateur Radio Blog for
more info on the Parasaki transceiver.

Click here to go to SolderSmoke Daily News for
more info on the Parasaki transceiver.

Attenuator of 20 dB to WSPR with 10 mW

QRSS and QRPp is an excellent combination.
The reduction in speed, can be translated directly into a reduction in power.
So if you are bored with 1 watt, then try this simple attenuator, which is built with good available E12-series resistors.

The input power of the attenuator is 1 watt, if the maximum power dissipation of each resistor is 1/2 watt.

This attenuator can be used to WSPR with very low power.
With the FT-817 on 1 watt, the power is reduced to 10 mW.
And with 500 mW the power of the FT-817 is reduced to 5 mW.
The difference between 10 mW and 5 mW will not be noticed, except if you look carefully at the remote S-meter of the receiving WSPR station.

7 Resistors and a switch is all you need for WSPR with QRPp with 10 mW. hi
But notice that the attenuator is also giving 20 dB attenuation while receiving.