HF Radios

While researching a piece of equipment from Ebay I ran across the Green Mountain Radio Research Company (click to open in new window). Dr Raab is an expert on RF power amplifiers and this is his consulting website. His article RF and Microwave Power Amplifier and Transmitter Technologies – Part 1 is a wealth of knowledge. It begins with a brief intoduction and history of power amplifiers and continues with linearity, efficiency, and RF power transistors. This article and the four parts that follow can be found on the High Frequency Electronics website. I am looking forward to finishing the series on PAs by Dr. Raab and digging through the extensive collection in the Article Archive of High Frequency Electronics.

RF and Microwave Power Amplifier and Transmitter Technologies  by Frederick H raab, et al.

Enjoy the read. 73


We have been having a difficulty with the VFO section of our BiTX20’s … I’m scratching my head and wondering why we’re off frequency but with the right spread. Enter Mario …

DUH, I never checked the values; we had discussed that there were no 560 PF NP0 caps in the kit, but I totally forgot. Mario asked, ‘where did you get the caps?’ Well, the builders had used the 56pf caps in the kit and it makes sense why none of them were on frequency.

We are making an order for 560 pf caps, but they will not be NP0. They will be very close, but we can’t find NP0 caps. It’s fine tho, the ones we are looking at have less than a decimal of a PF over a huge temperature range.

If you’d like to get in on that buy please contact Dave KC4ZVW or myself in the next few days and we’ll get you in on the pile.

I feel stupid, sorry I didn’t look at that, but at least we know where to get replacements.

Tracy N4LGH

This weeks Solder Social was a great learning experience!

Yeah, I’ve started calling our BiTX20 building group the Sewing Circle with Soldering Irons. A Soldering Circle! What’s the difference? A bunch of old bitties sitting ’round gossiping and trying to make ourselves sound important to one another. It’s a hoot!

We saw firsthand what a couple of picofarads can do to / for a filter. The BiTX20 transceiver front end has a nice bandpass filter that doubles as the front end filter for the receiver and the driver filter to the 5W PA. It’s a fairly important part of the circuit because if it’s not tuned well, performance will suffer on both receive and transmit.

First off, the documentation is a little contradictory to the markings on the board. In the photos of the board layout, the filter capacitors are marked 68pf. They are also 68pf in the supplied schematic. BUT, on the board itself, and in the written text, it calls for 33pf. In the text it states one could use either a 33 or 47 pf. We figured it was because of the tuning range of the inductors.

Some of us had already built the filter with 39 pf caps that appear to have been substituted for unavailable 33’s or 47’s. Maybe it was in the middle of the range?

Contemplating the filter

Dave KC4ZVW had his Filter / Amp assembly ready first so we applied power and checked voltages per the instructions – everything was within tenths of volts, fantastic!

Next we went about setting up the Tracking Generator and Spectrum Analyzer. Thanks to Tom KD4WOV for helping us get that set up. I’ve never done that on a filter before.

Wow, what a mess! It had huge insertion loss and a lousy pattern. But after fiddling with the caps a bit Dave found a nice compromise between pattern and insertion loss.

At last, a decent pattern with low insertion loss!

Mario KU5E had used 33pf capacitors and came up with completely different results. He ended up removing the 33’s and putting in the 39’s. What a difference! We spent quite some time fiddling with the pattern and insertion loss with the 33’s and were never able to get less than 15dB of pure wasted signal.

As usual, Vern Ki4SDY was an excellent host and generously provided some caffeinated beverages, although I’m not sure this group really needed any stimulants ;]

Best we could get with the 33pf caps!

Now this is a good pattern!

Once I finally figured out how to take a picture of the spectrum analyzer screen these shots came out pretty well. Next week I’m going to bring my desktop tripod so that I don’t have so many blurry shots. These screens sure tell the story. We were able to get these tuned to with a dB or so insertion loss.

Overall the filter looks like it has better than 20 dB overall rejection, with less than 3dB insertion loss in about a 100 kHz passband. We don’t think we have this filter optimized – I will likely try several values there. Perhaps a variable?

a higher resolution shot of the pattern

Everyone is invited to our weekly Solder Social at Verns barber shop in Longwood, near the intersection of Ronald Regan Blvd and Church Street. Vern is the second door east on Church.

Hope to see you all there next week!

Meeting #1

This was held on 30th of Nov 2009.

Talked about tools needed to assemble the project, soldering and the assembly instuctions that we are going to use.

I mentioned that the discussion list was on YahooGroups which can be found from here.

Learning about tools


Other pictures


David — KC4ZVW

KC4ZVW, Dave, commented on my last post and ask some basic questions that started me thinking. Does it work? It leads me to why I am building the BitX20. I have a strong desire to learn electronics. This project allows immediate feedback on each section that is completed.

Crystal Radio

When I completed the audio amp, I needed to find an input source to check it out. Travis, my son, and I built a crystal radio with an audio amplifier circuit. This Fox Delta design is made to learn with and has the ability to separate the audio amp from the crystal radio. I fed the signal into my BitX and it worked. I have already received the local AM station through my Bitx. DAVE, it works.

Then I built the Mic Amplifier. By jumping a signal and powering the audio amp section I was able to hear my own voice through my small amplifier setup. Yes it does squeal when you get the mic near the speaker just like the real audio setups, and yes I played with all the feedback noise I could create. That worked too.

I have completed all soldering on the radio. A radio is an oscillator that allows us to throw electromagnetic waves through the air and catch them at a different location. This radio has two oscillators. One is a carrier oscillator that is a 11.00001 Mhz, and the other is a variable oscillator, VFO, that is close to 3.0 Mhz. When combined, the frequency that adds up is near 14. Mhz, the 20m Band! Well , there’s a problem Dave. First, I have both signals oscillating and measurable although the VFO is only variable by 3.5 Khz. Not enough to qualify as a VFO. On this radio I will need a couple of hundred Khz to cover a portion of 20m phone band. I soldered in a different variable resistor and air variable capacitor and the oscillations stopped. I will go back on that change. Second, I am unable to find the 14. Mhz signal. I have unbalanced the modulator to let the carrier signal through to the mixer near the VFO and where I feel it should be, it ain’t.  There I is. Stuck again.

Dave, does it work? Wonderfully! I am learning radio theory, trouble shooting, soldersmoking, and burning my fingers. LIFE IS GOOD. Oh… the radio, well I’m sure it will work when I am done. 73


Bitx20A Alignment in Progess

Bitx20A Alignment in Progess

Hello, I have been graciously allowed to post some thoughts about my ongoing relationship with a Bitx20A.

Where to begin… my name is Robin and I am a homebrewer. It all began when I found out you could build your own and learn about how a radio really works. No more black boxes with multiple-multifunction knobs and buttons for me. I was completely innocent, without a clue of the power this would have over me. Before I knew it I was a frequent-flyer at SkyCraft. Late night sessions at the dining room table with smoke wafting about as I ponder the perplexities of mixed oscillator signals filtered through polycrystalline mineral wafers. I still remember the joy of hearing the audible oscillations of my first Colpitts, but I digress. Consider the consequences, it is not for the faint of heart. If this is the path you choose, know what lies ahead. Capacitors that disintegrate before your eyes, signals that disappear after just a small tweak, and the frustration of “dang, I followed the directions and it don’t work”. I warned you!

That said, I will post a few photos and discuss my Bitx20 from time to time. I want to thank Tracy for his inspiration on the Tech Net and David for his Web expertise that allows me to be here. 73


Well, the BitX20 project is officially launching. Patrick KJ4JTK is keeping the list of participants so contact him to get involved. His email is pberry26 at yahoo dot com.

Here are the basics –

This is a course designed to teach the General Class Amateur Radio License theory by building a basic 20 meter SSB radio, illustrating the electronic theory required to pass the General Class License Exam. All sections of the General Class Exam will be covered in the course and the exam will be administered at the end of the course. The successful student will emerge from the course with a quality SSB radio and the license to operate it.

Registration must be in by October 31st
The course starts November 30th

Price for non-members is $200 which includes one year membership.
Price for existing members is $185 with lifetime members at $175. “Audit’ courses are available at $135 each for either the General Class License only or the BitX20 Radio only.

Meetings will be on Mondays and Thursdays from 7pm-9pm at KI4SDY Verns shop located basically at Ronald Regan Parkway and State Road 434. Holiday dates will be skipped.

There is a list of suggested tools to successfully build the radio. ‘Loaners’ will be available on a limited basis. There will be test equipment present at some of the building sessions.

This is sure to be a fun and productive project. Those participating on any level should grow in the knowledge of how SSB radio’s work, and a better understanding of basic electronic theory.

Q/A sessions will be handled on the Wednesday Night Tech Net as well as the Monday and Thursday sessions.

Contact Patrick to get on the list. Once registration is closed we will accept payment for the course and project.

Completion of the class / project will be just before the Orlando Hamcation and we will have a ‘Brag Booth’ there.

Feel free to ask any questions of myself or the appropriate parties listed here. The Wednesday Night Tech Net is always a good place to ask questions about this class and project.

Tracy N4LGH


Today I received my FT-857 from Yaesu Repair. $160 in damage from a low battery.

I am going to be putting either a low voltage cutoff switch in my power line or put a dedicated battery in the trunk. I am leaning toward the latter with a charger circuit that can put the battery online for charging but take it out of the car circuit when the key is off or the engine is starting.

I’m not sure if I’m going to have the charging circuit in all the time or actually kick in at a minimum and kick out at a maximum voltage. I’m going to read more and take advice from the experienced before I do it.

In the mean time, I’m leaving the ‘817 in the car as it runs from 10v!

I’ve more or less completed my small loop antenna to my satisfaction. I’ve also updated the pictures and added a good edge on picture of the completed capacitor.

Reception is great, transmission is not as good as I had hoped, but not too bad.

I’m still experimenting with the height — it seems to work well at any height above about 4 ft, but has better local range up higher and less noise down lower. I need to do some directionality experiments.

I’ve gotten use to the tuning; it would be nice if I had better control of the speed.
Perhaps a good next project would be to make a pulse width modulation controller for the tuning, or make an interface to my ic706 to handle auto-tuning via the external tuner interface.

It’s hard for me to tell if anyone cares, so this will probably be my last post about this antenna unless people express more interest (via the net, comments, email, etc.).  Unless I get around to building either the PWM controller or the automatic tuner interface, which isn’t likely in the next month or so.

As I have mentioned on the net, I’m working on building an HF small loop antenna, as described in the ARRL Antenna book, chapter 5.  The target is 80m, but the more bands I can cover the better. The antenna I intend to build should be close to this:

Diameter: 6 ft
Circumference: 20 ft
Tuning capacitor value: 28 – 380 pf (for 30m-80m coverage)
Tuning capacitor voltage: 7kv – 10kv for 100w xmit

Since the trickiest part of this antenna will be the tuning capacitor, I’ve started there. I have not seen any air variable butterfly capacitors for a while (I saw one 2 years ago at hamcation, but I didn’t know what it was at the time), and vacuum variable versions cost $150-$500 on ebay (I saw a broadcast rated one for $3k), so I’ve decided to try building one.

The parts I’ve found at skycraft are:

  • some brass 2″ bolts
  • 1 mm plastic sheets (for dielectric between plates)
  • 0.7mm fiberglass single sided PCB
  • 1.3 mm plastic washers for spacers
  • 4mm brass nuts

I’m cutting the PCB into alternating stator and rotor plates, layered until I have enough capacitance.  The rotors are spaced by the 4mm brass nut (which I solder to the rotor and then tighten to the brass bolt).  One unit of the assembly looks like  stator – dielectric – rotor / plastic spacer – dielectric.

The nut passes between the stators and through both dielectrics and touches the fiberglass side of the next rotor.  So, each nut/rotor is 4.7mm (nut + pcb).  The stator side is 0.7 + 1 + 1.3 + 1 = 4mm leaving about 0.7 mm of air gap around the rotor for it to turn in.

I’ve drawn a nice diagram at http://www2.mmae.ucf.edu/~ssd/capacitor/ which also has a log of measured capacitance which I will update as I add plates.

Currently, I’m up to 13 plates for about 280 pf.

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