Bob pairs his ATS-20 with the Zachtek “Flea” CW transmitter

Many thanks to Bob (K7ZB) who shares the following guest post:

ATS-20 configured with Zachtek “Flea” CW transmitter

The ATS-20 in HF CW Transceiver Mode

by Bob Houf (K7ZB)

I picked up an ATS-20 last summer and played with it on SWBC and the ham bands but found the telescoping whip antenna to be marginal.

The unit I purchased from Amazon turned out to be solid: no problems have surfaced after 9 months of intermittent listening. By default, I have enjoyed it primarily listening to FM in my office.

When I used my long wire antenna, the performance on shortwave greatly improved – easy copy of DX and the value of the receiver began to impress me.

Recently I came across a Swedish ham who co-developed a line of radios covering a broad range of WSPR and associated designs built to a very high standard.

Already having a WSPR setup I was intrigued by a very low power CW transmitter that Zach co-developed with KB9RLW which puts out 300mW on 40, 30 and 20 meters at a price point that is less than the ATS-20, and – most interestingly – the design of the radio allows it to work in transceive mode with the receiver by providing a T/R switch when used with the proper SMA-BNC cable arrangement.

I bought a “Flea” and configured it with my ATS-20 then used an external antenna tuner to make sure the transmitter had close to a 1:1 SWR.  The “Flea” advertises that it is robust to high SWR and looking over the schematic that is evident, but for the sake of a trial I went with the ATU.

My wire antenna is up on the second-floor balcony of our condo here in Arizona, runs about 47 feet out to a tree and is up about 25 feet above ground between the roofs of two condo buildings.

The feedpoint of the antenna has a 7-foot counterpoise that drops straight down and is fed through two common mode chokes to cover the 40 through 10-meter bands.  The chokes are designed according to the cookbook available online from K9YC [PDF] and even when running 100 Watts from my HF transceiver, I have no issues with circuit breaker tripping or EMI.

I made my first contact with this setup to a SOTA station in California on 30 meters and received a solid signal report of 579.

The transceiver arrangement worked flawlessly and with some experimentation I was able to use the ATS-20 seamlessly and learned a few things to enhance its performance.

The bandwidth of the ATS-20’s filters in CW is very good – a key to spotting your transmit frequency (zero-beating) with the “Flea” is to open up the filter to 4KHz until you find your transmit frequency then narrow it down.  The “Flea” uses an innovative tune mode developed by KB9RLW that dispenses with the usual rotary encoder and since there is no display on the transmitter you use the ATS-20 display which works quite well.

The sensitivity of this receiver with a decent antenna is truly remarkable; with the filter opened up to 4KHz there were layers of CW signals heard on the bands. I should note the sunspot number was helping things on 20 meters.

One thing that would improve the receiver performance would be to provide more attenuation beyond its current max setting of 20.

The BFO is very useful and gives quite a broad range for finding and zero beating stations.

Assuming you don’t have a QRP transceiver – like the QRPlabs QCX+ – but do have an ATS-20 or similar receiver, the “Flea” opens a new multiband world for these low-cost Si4732-based receivers.



5 thoughts on “Bob pairs his ATS-20 with the Zachtek “Flea” CW transmitter”

  1. Great report Bob!

    QRP Stealth Ops, ‘El Condo’ style… I like it!

    We’ve owned this house for twenty years, but I live in an HOA with a lot of Left Coast refugees moving in, so stealth it is… It’s all part of the thrill, being ‘sneaky.’

    de W7UDT (dit dit)

    1. I have a friend who also lives in expensive home with HOA, the place is well kept. And as you guessed it no outside antennas. He found a flag pole that is an Off Center Fed vertical, has insulator at the bottom and the pole is metal and the antenna. Does come with the flag hardware. He has worked many countries. Not QRP and the antenna is ok on 80 & 40m, but does do well on 20m and above. It is 24 ft long. There are ways to get up an antenna. 73, ron, n9ee

  2. Hi.
    I purchased a CWFlea which I paired with my Malahite. Everything works fine. But I would like to mount a 5W amp for the transmitter and I’m afraid that the power emitted will damage the input stage of my receiver. What do you think ?
    Jo – F5NFB

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