An Antenna for the High-School Direct Conversion Receiver (and Next Steps in the Project)

We have reached the point where we have to decide on an antenna for the high-school direct conversion receiver.  It needs to be simple and easy.  It needs to be something that students can easily install from a bedroom window in an apartment or a town house.  

We thought about an End Fed Half Wave, but 66 feet of wire seemed to be too much, and the EFHW would require both coax and the construction of a transformer.  That seemed like too much. 

So here is what happens with just 33 feet of wire (1/4 wave on 40 meters), with another 33 feet as a counterpoise.  I found that the counterpoise worked just as well spread out on the bedroom floor as it did hanging out the window along the outside of the building.  As you can see in the video, the counterpoise is really necessary with this kind of antenna.  It makes a big difference. 

We know that the students could have dispensed with the counterpoise by connecting the copper clad boards to a cold water pipe, but that might be difficult for them.  So we went with the counterpoise. 

After the antenna demonstration I ramble on a bit about the high-school construction project, and where the students could go from here. 

The Awesome Homebrew of Will KI4POV

 

Will's homebrew station is really something.  The rig (it truly qualifies for this term of praise) is amazing all by itself  (see below), but a look at Will's QRZ page reveals other ingenious inventions and techniques:  There is the clock made from panel meters.  And the method he uses for making aluminum project boxes.  He even made an N0WVA regen receiver.  That's the one I used in my ET-2.   Fantastic.  

Will really needs to share his homebrew skills with others.   I hope he is soon in a local high school teaching students how to build things.  Thanks Will!  

Bill and Pete,

Just wanted to send a note to update you on the latest projects here. The last time I emailed, I mentioned wanting to build a superhet, which you (rightfully) discouraged, pushing for a DC receiver.

Well, I finished the superhet at the end of last year. I had most of the receiver working long before then, but got bogged down in an AGC system. The final receiver is a 5 band si5351a controlled single conversion superhet loosely based on Todd, VE7BPO's design with several modifications. I used a 9 Mhz IF instead of 4, I used a digitally controlled LO and BFO, and as mentioned, I added an AGC system, which ended up being the most challenging (and most interesting) part. The final AGC system I ended up with used the detector and amplifier side of Wes Hayward's "full hang" AGC from SSDRA, but I didn't have the IC IF amplifiers with variable gain, so instead, it fed a PIN diode attenuator circuit to control the IF gain. The final result worked great, but I nearly pulled my hair out getting it to work. I originally intended to build the receiver for 40 and 20 meters, but it ended up covering 80, 40, 30, 20, and 15 since I used the filter relay board from QRP Labs which had 5 slots.

After I got the receiver running, I decided I needed a matching transmitter, so I  built up a simple CW transmitter to match. It uses an si5351a VFO driving a 74HC04 hex inverter as the buffer amplifier. The trick here is that by driving all the inverter gates in parallel, the output impedance is ~14 ohms broadband, suitable for driving a BJT PA without any need for matching transformers. The PA is 3 2N2222s in parallel with heat syncs putting out about 2 watts from 80 - 15.

The part I'm most proud of is that I have the arduino for the receiver connected to the arduino for the transmitter through a serial line. The receiver sends it's current frequency to the transmitter so that the transmitter can track the receiver's frequency as you tune (like a transceiver). I'll attach a few pictures of my homebrew station below. The transmitter is on the left, receiver is on the right. The box on top of the transmitter is my homebrew keyer. Next up on my build list is a solid state T/R switch.

Bill, I've enjoyed following the updates on the school DC receiver build. My local club is wanting to do some youth outreach, and I'd love to get them involved in building. I'm the only builder in the club though, so I don't know if I personally have the manpower to make it happen. Also, thanks for the recent info and pictures of Cuban homebrew rigs. I grew up listening to Arnie Corro, so I love seeing their resourcefulness and ingenuity. Makes me want to take apart some old electronics.

Pete, I'm enjoying following the 10M SSB project. With the uptick in propagation, I've been bitten by the 10M bug, and I'm thinking a 10M rig may have to be on my project list for this year.

73,

Will - KI4POV

KI4POV's Clock

KI4POV's N0WVA Regen

Update on the High-School Direct Conversion Receiver Project: Board #4 Completed

One of the receivers. (l to r) AF, PTO, Mixer, BP filter

The students continue to make great progress on their direct conversion receivers.  Yesterday they were enthusiastically sitting down to workbenches, building Manhattan-style circuits, working directly from schematic diagrams.  It was really impressive.  They are building receivers. 

This week we had two sessions of about two hours each. We introduced the final board:  the audio amplifier.  Many of the students began work on this amplifier; others were catching up with work on boards presented earlier. 

The AF amp is their most challenging board:  It used 14 Manhattan pads and about 26 components.  We warned the students that amplifiers often aspire to be oscillators.  We told them to pay attention to layout, and to keep their leads short.  

One of my builds, and a Manhattan board pattern

At first, the students just built the first stage on the AF amp board. They tested this, then moved on to build the other two stages.   

By the end of Friday, two groups had completed the build of the AF amplifier board.  

We think there are about 13 receivers in production.  Some are near completion, others will need more work.  

On Thursday of next week those teams that have completed all four boards will put the circuits together and will test the entire system.  They will then add all needed front and back panels and socketry. 

We really want the students to complete as many of these receivers as possible.  Exam season and the end of the school year is approaching, so we have to get this done.  We will remind students that they don't want to that person who ALMOST finished a project!  We will urge them to GET IT DONE!   They can tweak it and mod it later.  This kind of tweaking and modification is part of the homebrew experience. 

We have been presenting awards to the students who are first to complete each stage:  The winners of the PTO board competition got a copy of SolderSmoke:  Global Adventures in Wireless Electronics.   Those who won the mixer competition got a W1REX Hamfest Buddy transmitter.  Thanks Rex!  And this week we presented an award to the students who were the first to complete their bandpass filter.  You've heard of the Tony, the Emmy and the Grammy?  Well, we presented "The Torry"  (from Toroidal).  The trophy was made from a toroidal winding tool made in Alaska by KL7FLR.  I explained to the students who had made it.  Thanks Paul! 

"The Torry"

Homebrew AM from Cuba

Jose CO6EC sent us more information about homebrew rigs built in Cuba.  The transmitter above is a thing of beauty.   I am really glad that Jose has held on to it.  Thanks Jose! 

Jose CO6EC today

Jose CO6EC writes: 

This is another work from that time:  an AM and CW transmitter of about 100W of power, with an on-board modulator.  It was taken with some modifications from a Handbook from the 50s.  Here goes:  it was built with what we had on hand at that time, it still exists, I keep it as a relic of those years.

It used combined Soviet and American tubes in the RF sections:  Soviet 6P9 and (2) 6146.   The modulator used Soviet 12AX7, 12AUT and (2) 6P7. The 6P9 works as a crystal oscillator, and in  case of using an external VFO it works as an amplifier and doubler or tripler to obtain outputs in the  160,80,40,20,15 and 10 Meter bands. 

The VFO was also taken from a 1949 Handbook if I remember correctly, but I don't have any literature on that, as you can see in the photo it has 5 5u4c rectifier valves, VR150 voltage stabilizer, 6v6 output, another 6v6 as a separator and a 6 )I(4 (60I94?) Soviet, in the oscillator.  The stability they achieved in those years is incredible,  I could communicate with stations that were on LSB and if I didn't tell them that I was on AM they didn't notice. 

  As a receiver I used an old   Soviet AM and CW receiver, used by the Aviation HF stations   of the 40s, which no longer exists hihihihihi

I'll tell you how I tuned all that good ftuff:  First I put the receiver  in CW to beat the signal of the stations in SSB.  After hearing them clearly, I removed the oscillator of the receiver  and connected only the VFO of the Transmitter, and beat the signal with that of the receiver until I heard the other station  clearly, then I put the transmitter to work and was ready to communicate.

There were many communications made with that station, even internationally on phone  and CW. 

Today everything is easier because with transceivers it is not necessary to go through all that work, but it is always good that those who did all this work know how radio was built and made.

 73   Jose CO6EC

Obviously the transmitter

The two pictures above must be the VFO.  It looks like the VFO in the 1947 ARRL Handbook. 

I wonder what handbook this was?  Spanish language. 

More Cuban Homebrew from the 80's and 90's -- A 160 Meter AM (Controlled Carrier) TX with Soviet 6P23s

 

160 Meter Transmitter

Jose CO6EC writes: 

I will be sending you some work of homebrew from the 80's and 90's when practically everything was manufactured by Cuban radio amateurs

The photos are of a Modulated Amplitude TX for the 160 Meter Band, about 25W of which several were made.  The final tube  was modulated in many cases by a 6DQ6, 2E26 or other similar ones that were very abundant at that time. 

This one in particular is  made with valves  6BH6 for the VFO, 6CL6 for the amplifier step and 6P23 Soviets in the final part, the modulator was made up of a 12Ax7 microphone preamplifier and a 6BM8 that came out through the cathode to the screen grid of the 6P23, in this way the carrier was controlled by modulation what we called  Carrier Control as there was no voltage on the grid at the ends there was practically no carrier  in the air which gave the impression of transmitting in SSB. hihihihihihi

For those who did not have a communications receiver, a conventional radio was adapted to receive that band, which in many cases was a VEF-206 of Soviet construction, to which an oscillator was adapted to beat the signal and get exactly on frequency. This was very popular here in the late 80's and early 90's. 

Thanks Jose.  We look forward to learning more about Cuban homebrew. The way in which radio amateurs got on the air with gear that they made themselves using the limited parts available to them is really interesting and admirable. 73  Bill