The Big Kahuna

 

This web page has nothing to do with surfing or the Hawaiian Islands as the name of the rig might suggest. Instead the title of "Big Kahuna" is to recognize that the color TFT display is large, like 320X240 as opposed to the smaller 128X128. Thus a lot more information can be presented on the display face. The available color palette numbers some 256,000 choices, or roughly 2^18. However some of the greens and yellows make you feel queasy.

This rig was designed to be a bit more than QRP (17 -20 Watts) and barefoot with a beam has worked DX on 20 meters reaching out to the 5000 ~ 6000 mile range. But it also was intended as a driver for a high powered linear amp, where having 800 watts output can put you in the league with the big dogs. Audio and signal reports have been excellent! So I am pleased that it works well, and is a homebrew design of my own and not a copy of other rigs. This rig has soul!

There are many creative hams out there in radio land building rigs using some of the latest (and cheap) technology. The Bitx has been tremendously impactful as evidenced by the sheer numbers being successfully built. But there are other very respectable designs and this is one of them! Later there is a link to the Arduino code.

The web pages will start with an overall description of the rig and then detailed schematics and photos will be provided so those who would like to have one of the alternate designs to the Bitx and can easily replicate this rig.

To begin my quest of "different than a Bitx", I initially started with a bit of noodling about what a requirements listing might look like for this new rig. Shown below is my matrix of "noodle" ideas. If it was a case where all of the factors were met with the Bitx design, then a new radio would be a non-starter. But after finishing the listing (and considering some of the constraints like not a single board or that real homebrew skills were required), it was pretty obvious that I would proceed with the Big Kahuna.

 
Feature
Big Kahuna Bitx40
1
Color TFT Display (lg)
Yes
No
2
USB/LSB Select
Yes
No
3
CW
No
No
4
2 Band Operation
Yes
No
5
5 Band Capable
Yes
No
6
S Meter & AGC
Yes
No
7
20 Watt Output
Yes
Yes
8
Pre-Built
No
Yes
9
Bilateral Design
Yes
Yes
10
Single Board
No
Yes
11
Skilled Builder Req'd
Yes
No
12
TUNE Function
Yes
No
13
Linear Amp Switching
Yes
No
14
MOX Operation
Yes
No
15 Si5351 CLK Generator
Yes
Yes

You have to ask why would you build the new rig and it boils down to the additional capabilities that are included in my design and not as an after thought. These include:

  • USB/LSB Selection -- Nice if you want to have a choice like PSK on 40 Meters
  • TUNE capability. There are no TUNE provisions in the Bitx
  • Color TFT Display. There is just something special about color at 320X240
  • Five Band Capability. The Arduino Sketch has this as standard.
  • Commercial Filter. Either the GQRP or INRAD Model #351 9.0 MHz filters will work
  • S-Meter + AGC Circuit. Not an add on.
  • Linear Amp Switching and MOX operation.
  • Not mentioned previously, but possible using the 3rd Clock --CW operation!

Based on my matrix evaluation it was a go to proceed. Let us start with a transceiver "Block Diagram" which is presented below:

Big Kahuna Block Diagram

The real key element in this rig, aside from the Arduino/Si5351/Color TFT is the bilateral amplifier stage which is a circuit developed by Plessey for use in man pack radios and was found in EMRFD. Having come from EMRFD should ameliorate those who must see circuit pedigrees before accepting that something does indeed work and work well.

What could be more simple than two inexpensive transistors and a handful of likewise inexpensive parts. Essentially the circuit amplifies in two directions pending how a circuit element is connected to 6 VDC or to Ground. I now run this circuit at 8 VDC and while the Big Kahina uses relay switching for the signal change, my latest transceiver on the workbench uses FET switching to connect either to Ground or 8 VDC. The stage works well into the HF region (40 MHz) and is good for about 17 dB of gain. [A quote from EMRFD --not Fake News!]

For those purists who only comment (usually negative) but do not actually build things, you could substitute the Termination Insensitive Amplifiers from Hayward and Kopski using a lot more parts for about the same level of gain. Your choice. This circuit has been built with both leaded and SMD parts. I believe beause the SMD version (since I have a CNC Milling program for that version) can be exactly replicated gives a slight edge over the hand wired leaded version. Shown below is the SMD version and you can easily spot some of the circuit values. While not the most compact SMD -- I can actually solder parts to this board.

 

Since many home brewer's are terribly impatient, I will now provide links to my rig building blocks so you can just skip ahead and then when you run into a problem you can come back to read the detailed information on how things work. Just my opinion but based on factual data: When I receive the YCDW email (your circuit doesn't work email) --about 99% of the time the builder simply forged ahead and missed some critical data in the write up about what needed to be observed during the construction process. Fools rush and usually smoke the parts!!!!!! Read The Manual.

Noteworthy is that I usually build my projects in block modules starting at the back end and each block is tested after being built. When a block is completed it now becomes a part of the test system. So if after adding a block the circuit is in op you know the problem is with what has just been added. Thank You Heathkit!

 

  Circuit Block Module
1
Audio Amplifier & Microphone Amplifier Stage
2
AGC and S Meter Stage
3
RxTx Mixer, IF Amp and Product Det / Balanced Mod
4
Bi-Directional Amplifier Stage
5
Band Pass Filter Board
6
Low Pass Filter Board
7
Power Switching Module
8
Arduino/Si5351/Color TFT
9
Miscellaneous Wiring and Final Assembly
10
Arduino Sketch for the Big Kahuna

Basically my design is for a two band transceiver based on a single conversion at 9.0 MHz, using a commercial filter available from the GQRP Club Sales. These can be had for around $25. The two bands that were selected for operation were 40 and 20 Meters. This seemed (and has proven to be) the best choice given our sunspot activity is literally at a minimum.

But embedded in the code are selections for five bands and simple grounding of one of the five Arduino pins will put the transceiver on to one of those bands. In fact the code could be changed so that the five 60M channels could be selected. So many options are available in this feature rich rig!

Two bands however simplifies switching of the frequencies as well as selecting the appropriate Band Pass and Low Pass Filters. This is done with a single DPDT toggle switch. One half the switch tells the Arduino to change frequencies while the other half enables the selections of the appropriate BPF/LPF combination. Flip a switch and automatically a new band. Switch to the appropriate antenna, hit the PTT and you are on the air!

If the builder would like to have all of the five bands then a two pole five position switch is all that is needed. [Plus the Band Pass and Low Pass Filters for each band.]

The unit can be switched between LSB/USB and this is done with a simple SPST toggle switch. The code makes an adjustment for USB/LSB carrier frequencies so that the display is correct for either mode.

The power output using a module from Communications Concepts Inc. is 20 watts on 40 Meters and about 17 watts on 20 Meters. With my homebrew 3CPX1500A7 which operates only on 20 meters, 800 watts out is easily attained. Couple that with my two element Mosley beam and you can run with the big dogs --and I do! Imagine my glee when I can say I am running a total homebrew station. The CCI amp needs but a few milliwatts for full power output. Following the Band Pass Filter is a single 2N3904 which drives the CCI module to full power!

Some refinements not included in the Bitx is a "TUNE" function. I really don't know how you are supposed to tune up a Bitx for maximum performance without the use of some sort of external tone being generated and fed into the microphone circuit. My approach uses the Arduino to generate a 988 Hz tone that is pulsed and of a timed duration. The output of the Arduino is a Square Wave and thus a three stage RC filter is inserted between the tone pin and the balanced modulator. The RC filter makes the waveform appear sinusoidal.

In this rig, since I have five pins reserved for band switching, a slightly different method was used for TUNE. In other rigs I built, simply pushing a momentary push button starts a three element process. Step one puts the rig in transmit, Step 2 creates the pulsed tone and Step 3 returns the rig to receive after the timing period is completed. All automatic. In this rig I added a panel mounted toggle switch that trips the PTT and in essence gives you MOX. The Tone PB is activated and the timed pulsed tone is sent to balanced modulator. With the timing cycle over simply return the MOX switch to receive. Actually I found the MOX switch kind of cool as you don't have to hold down the microphone PTT in normal SSB QSO's.

 

 

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