SB-200 input impedance correction on 10 and 15 meters

I had known that both the 10 and 15 meter band input circuits on my old Heathkit SB-200 linear amplifier were not well matched to my transceiver.  Using this amplifier with a newer radio, would cause the transceiver to fold back on these bands with high SWR.  Referring to the schematic we see that these employ an L-network to attempt to match the input impedance to standard 50 ohm load.

Going through the mathematical exercise for a PI network, which I won't trouble you with here, it stands to reason this would probably work better just like it does on the lower bands.  This more sophisticated matching network filter can have more loss, but that's not something I was worried about.  The limited number of turns on the 10 and 15 meter coil meant that they required some fairly small capacitors to attempt to make the matching come out correctly.  The problem is that those size capacitors are about the same value of some of the stray capacitance I believed to be in and around the wiring.

The plan was to double the number of turns and increase the capacitance required on the Input side of the network.  I then stumbled on this article by PA0FRI who came to the same conclusion.

I stripped the 10 and 15 meter coils from their windings and re-wound with 1mm diameter magnet wire.  Scrounging through the junk box yielded the capacitors that I calculated and were confirmed by PA0FRI.  I didn't have any tuning caps available, so trial and error yielded appropriate values for the output capacitors which had to factor out the stray capacitance in the wiring.

The trick was how to validate and tune the slugs in the inductors.  I've lost the source from a forum post on eHam, but the procedure is to simulate the resistance of the tubes per their data sheet.  So we temporarily place a 220 ohm resistor between the filament lead and then to ground on the chassis.  While manually pressing in the transmit / receive relay an antenna analyzer is used to measure the impedance at the input connector.  After few tweaks of the slugs both bands yielded acceptable impedance at less than 1.5:1 SWR reading.

The final results are below:

Amateur Radio - WARC band fan dipole

Welcome back to the Cly Institute for Radio Repair subsidiary of the Imp Barn. The institute wants to be capable on as many bands as we are licensed to operate on.

The WARC bands are three portions of the shortwave radio spectrum consisting of 30 meters (10.100–10.150 MHz), 17 meters (18.068–18.168 MHz) and 12 meters (24.890–24.990 MHz). They were named after the World Administrative Radio Conference, which in 1979 created a worldwide allocation of these bands for amateur use. The bands were opened for use in the early 1980s. They can sometimes be ignored by ham operators, but possess good opportunity for DX under the right conditions. My manual tuner works HARD to make my transceiver happy on these bands with my existing equipment for very poor efficiency, and very few contacts.

In researching the availability of antennas for these bands I found very little. Verticals are available from the usual sources, but the cost is prohibitive. (I didn't want to include 60m in this design either since my radio doesn't do 60.) The bands are nice and narrow bandwidth with little need to strive for obtaining wide bandwidth capability in a design. I normally run a commercial "fan" dipole which Alpha-Delta refers to as a "parallel" dipole for 80, 40, 20, 15, and 10 meters. What was needed was a simple wire solution. I was inspired by an article by W4DAN for this fan dipole design. The solution was simple, create a fan dipole for the WARC bands using first principles where the calculation is 468 / freq in Mhz = total length in feet for a half wave dipole.

 To reduce interactions between elements, I spaced them 6 inches apart. This may be excessive, but works fine. The center is an old 1:1: balun I found at a hamfest.

With the longest element of just over 23 feet in the center, the next longest on top and the shortest on the bottom.  A Dacron rope makes up the difference between the 17m and 12m element to reach the last spacer.

30m just over 23 ft

17m just under 13 ft

12m just over 9.5 ft

Here's a shot at dusk before the tuning process got fully underway.  We did all this work by flashlight in the dark.  This has few advantages, but at least the neighbors don't stare.  Its SCIENCE folks!

When tuning this antenna, I put the center up on a thin bamboo pole supported by my son's wooden swing set, and supported either end so that I could reach up at the end and grab it.  After getting the wire in hand I could walk hand-over-hand bending the bamboo pole down to reach the end of the element for tuning.  This was rather more like an Inverted-Vee arrangement which will result in slightly shorter lengths and will be ideal for mounting where there are no high objects to tie to.  At the recommendations of my elmers, I started with the 30m elements and tuned them to under 1.5:1 SWR.  Progression was to adjust 17m and then 12m elements.  I noticed little interaction between those elements while tuning.  I did jump back and forth once between 17m and 12m to get the tune just right.  The bamboo pole worked well for a temporary tuning setup.  A final mounting location is yet to be determined.

Using my VK5JST analyzer, we were able to get the tuning reasonably close.  I discovered that a simple technique to keep me from cutting too much off the elements.  It didn't seem to affect the tuning and I would encourage others to take advantage of this trick to enable you to re-tune an antenna in its final installation for perfect matching.

I folded the element back on itself and used a cable clamp to secure it.  The loop can be adjusted easily by loosening the cable clamp and re-tightening.  The element wire in the loop didn't seem to matter much, but I kept it to a minimum without too much wire involved in the loop simply for fine tuning.

Good high quality T-B ty-wraps were used to keep the spreading spacers taut with the 30m elements, and with the spreader near the feed point to keep everything aligned.  The ty-wraps can be adjusted slightly if needed but provide a good solid way of keeping the spreaders in place.  This is what Alpha-Delta uses, so why not!

Catch you on the WARC bands!

Amateur Radio - Building the VK5JST Aerial (Antenna) Analyzer

I've been wanting an antenna analyzer for a while.  The capital investment was a little too much to substantiate for a commercially produced unit capable of HF through VHF.  I stumbled across a few home-brew solutions, but none really seemed professional enough until I found the VK5JST Aerial Analyzer.  The device comes in a kit form from the Adelaide Hills Amateur Radio Society in Australia.  For under $125 USD shipped, it was almost too good to be true.  The kit consists of all the parts required and an enclosure.  The user experience is similar to the MFJ device which is more than twice the price.  All that is required is some sweat equity and some good soldering skills.  I placed the order and within a week I had a kit delivered to my doorstep.

Figure 1 - unpacked

The kit arrived complete with all the required parts, a high quality circuit board, adhesive labels, a battery holder and some assembly directions.

The first step was to lay out the enclosure, cut some holes, and drill some bolt holes.  The printed directions had a well dimensioned drawing, alas it was not to scale.  No worries.  I set forth laying out the front panel by using the board as a template, with an accurate metric scale to verify the measurements.  A set of drafting dividers provided pinpoint accuracy when marking the cutouts and holes.

Figure 2 - Laying it out

The double-sided board is very high quality with a great ground plane.

Figure 3 - cutting the holes

The tricky parts were making the square holes in the plastic project box.  This was done using a roto-zip bit in my Dremel tool.  Marking out the limits with blue painters tape gave a good contrast when routing out the shapes.  The holes had to be absolutely accurate to allow everything to line up.

Figure 4 - Cutouts

Figure 5 - Completed cutouts

The side adjustment for the tuning capacitor was the most difficult part.  I used a "flapper wheel" emory tool for the Dremel to smooth the shape of the cutout.

Figure 6 - Connector board to Main Board

The connector board is conveniently marked with centering tracks to aid in getting the boards to mate just right.  Following the directions, tacking the boards together first is critical before applying a drafting square to to insure a 90 degree match between the surfaces.  The first components to be added were the surface mount devices, which go on the trace side of the board.  These SMT components are very small and just a little heat is needed to put them in place while holding them steady with a pair of forceps.  It should be noted that reading the directions closely is imperative while positioning the surface mount transistor, since the label on the part goes DOWN towards the board.

 Figure 7 - Resistors

Each resistor was carefully verified with my trusty DVM (Digital Volt Meter) before soldering it carefully in place.  The leads could then be clipped off on the track side.  The directions recommend doing the smaller components first and then following up with the larger and higher components.  This makes very good sense so that the clearance of placing the smaller components is easier without the larger components in the way.

Figure 8 - Track side of the board with resistors

Adding the diodes and capacitors came next.  I was confused at first because the kit came with two extra 100nF caps.  I double checked the inventory count to make sure I wasn't missing something.  There are only the ones called for required and indeed the kit had two extra.

Figure 9 - Capacitors

The kit instructions were careful to mention that the caps should be tight against the board with no lead length above the board surface.  This reduces stray capacitance.  There is a note to actually leave the 68pF cap above the board surface for a better result and along with R31, R32, diodes D2, and D3.  I actually went back later and read the errata notes, and raised them up more.  I'll mention that again further on in the build report.

Figure 10 - Sockets

Adding the sockets for the IC's was a bit tricky while holding them flush and tacking one pin on each side.  Once they start going wampus its hard to correct it, so I made sure they were flush before soldering each pin in place.

Figure 11- Transistors

The FET's (Field Effect Transistors) that came with the kit were selected as matched sets, so the test circuit provided in the instructions was not needed.  The BJT's (Bipolar Junction Transistors) were also tested with the transistor checker gain feature on my DVM.  The kit noted that all are not created equal and the position of the pins should be verified on each device.  This was easily done after pre-bending the transistor leads carefully with a set of sharp needle-nosed pliers.  Adding the trimmer pots was also an easy task since they are all the same value, with the exception of the display contrast adjustment, which can't be mistaken since it has a different form factor.  The 6 multi-turn pots should be installed per the included assembly diagram.  This is important per the direction so that an increase in measured voltage on the Test Point terminal increases with Clockwise rotation and decreases with Counter Clockwise rotation.

Figure 12 - Inductors

The last tricky part was the inductors which were thankfully packaged separately in the kit!  Since I'm colorblind it was hard enough telling the resistors apart.  The inductors were also verified with my inductance adapter for my DVM.  This little adapter is a great tool.  I've had it for years and totally takes the guesswork out of inductors.

Figure 13 - Selector switch

Reading the directions for the selector switch was actually the least clear point of the directions.  I had to read it over and over to make sure I was doing it right.  To aid posterity, the ALPHA selector switch is marked with pins 1 - 12 around the edges.  Set the stop pin per the instructions at pin 6.  This is done by removing the nut from the shaft, and re-positioning the stop into the number 6 hole.  Looking at the selector Pin 12 goes to the 12 o'clock position with the antenna connector at the top.  I verified it all twice with the continuity checker before soldering in place.  The little wire inductor is a stretch across the center common pins.  I bent the switch pins and carefully fit the inductor and the small connector lead at the 10:30 position using a piece of clipped component lead.  Once again make sure everything is flush against the board before tacking the switch into place.

Figure 14 - Switch in place

Figure 15 - Tuning Capacitor

The tuning capacitor threw me for a loop at first.  The leads were coming out the wrong side.  You must remove the plastic cover from the back and put the leads out the bottom away from the shaft, before putting the plastic cover back on.  They are VERY thin, so be careful that they don't break when turning them 180 degrees.

Figure 16 - Mounting the Display

I don't trust double-sided tape, so I made four additional holes to mount the display.  The directions assume that you know which way is UP on the display.  The terminals 1 to 14 go down towards the selector switch.  I used 3mm bolts and spacers to set the display permanently in the front panel.  The last think you want is for double-sided tape to fail and short the display into the main board!

 

Figures 17, 18 - Display Mounted permanently

To my annoyance the wire that was intended for the display did not accept solder readily.  I tossed it aside and used some nice CAT-5 wire for the 9 connections from the display to the main board.  A few pins are common on the main board, which conveniently allowed for less wires to be run.  The pins 7,8,9,10 on the display were shorted with a bare wire and a jumper from 1 to 5 installed.

Figure 19 - Mounting Check

The board was then test-mounted to the front panel to check the spacing.  21mm listed in the instructions wasn't quite the right reference point.  I determined the actual distance based upon the thumb wheel clearance to the front panel.  The measurement was actually 21mm from the nut securing the bolt to the front panel with reference to the board's track surface.

Figure 20 - Test Fit

Patience and careful measuring paid off.  Everything lined up.  I used the soldering iron to melt the locating ring of the power switch into the front panel.  It was more easier than drilling a hole.  The selector switch shaft had to be cut off with a Dremel plastic cutting disk as it was far too long.

Figure 21 - Watch for smoke

While this photo doesn't capture it, the directions say to only insert IC1 and test it first.  The output was verified with the 'scope to insure that a nice sine wave was being generated at the correct specifications.  Setting the corresponding pot was also done.  The other chips are added per the instructions setting each corresponding pot.  Here's where the first problem arose.  I could not obtain a 5v full scale on pot P3.  I fiddled with the components associated with that circuit and discovered that if they were raised more from the board's ground plane an appropriate reading could be obtained.

Figure 22 - Success

While I didn't have exactly the right adapters to get a 50 ohm dummy load onto the antenna connector some quick scrounging in the parts bin yielded a workable solution.  A few more tweaks on the calibration procedure got a perfect 50 ohm digital (and analog meter) reading with 1.0:1 SWR and 0.0 reactance.  I was breathing a little better at this point.  This simple verification made me feel that the build was successful.  The front label could be safely applied.  The label had to be cut out to accommodate the display, which was done with a straight-edge and a razor blade.  The selector and power-switch holes were clearly marked.  I punched them through with a set of leather punches and used them to locate the label roughly.  The final precision location was done by the display cutout before pressing the label down permanently.

Figure 23 - Finished Product

This is a sharp-looking instrument.  The last build aspect is to add the battery pack.  Once again I don't trust double-sided tape.  The battery holder has mounting holes, and really should be bolted in.  Non-Skid feet on the back side of the instrument is probably needed to keep the instrument sitting level on the table.

The final test would be to compare the performance of the VK5JST analyzer against a commercial unit to verify the accuracy.  There are issues with the build however, and I haven't been able to resolve them presently.

Remember the adjustment of P3 that I could not obtain full scale?  I cannot obtain much more than half scale with the pot fully clockwise.  I went back again and this time removed the components from the ground plane side of the board.  After verifying them, I re-installed them on the trace side of the board.  There was no significant change here, so the problem must be elsewhere.  More study and research is needed on this subject.  This is what is happening.  I got a good N-Connector to BNC adapter so I could put a good terminating resistor on the instrument.  With some reasonable tuning on P3, P4, P5 and P6 the instrument DOES indicate a reasonably reliable reading of about 50 ohms across all the band settings (with no reactance).  A little fiddling with the P3 adjustment while the 50 ohms was installed, I could obtain a 50 ohm reading on the analog meter.  Feeling rather pleased with myself, I then swapped the 50 ohm BNC terminating resistor with a 75 ohm BNC terminating resistor.  The instrument reads approximately 30 ohms.  I'm defeated at this point on this subject and have to admit failure.

The second issue was discovered while tuning around in the 10meter band.  The display cuts out to zero and does not indicate any frequency above approximately 21 Mhz.  However, I know the oscillator continues to function, since I can detect an RF signal from the unit once the display goes to zero frequency reading.  Strange.  Something had to be wrong.  This device has rave reviews and should work!

At this point, I consulted with VK5JST Jim, the designer of the analyzer.  Jim got back to me right away and developed a checkout plan for testing a few points in the circuit.  At first we couldn't make sense of why the output of D3 diode was low.  It should have been approximately the same as the voltage at TP1.  I was still unable to get the Full Scale calibration out of Test Point 3.

I double and triple checked many of the components around the IC5 detector sub-circuit.  All the components were in tolerance.  Still it was not functioning.  Jim had me do another test as he suspected that TR5 transistor was mis-behaving in a way that it was oscillating in the UHF range.  It turns out that TR5 was not the exact part used in the original design.  The exact part should have been a Phillips brand BRF96.  Whatever part was included in the kit was not exactly a Phillips component and was not behaving as expected.  The use of a well calibrated finger was used to stop the UHF oscillation by touching the emitter lead of TR5.  This clinched the situation and we knew what had to be done.  Jim instructed me to ignore the errata note about re-positioning the components on the opposite side of the board, and put them back on the ground plane side.  I did so and at the same time lifted the lead of C14 from the emitter terminal of TR5.  To simulate the "calibrated finger" I soldered a test point pin to the board in place of where C14 would have been soldered.  This upset was enough to stop the UHF oscillation of TR5 and allow it to operate normally.

I then went through another calibration process, obtaining FSD on TP3 adjusting P3.  P4 was adjusted once again for 4.4v at TP4.  TP5 and TP6 obtained 2.06v easily.  I ended up tweaking P5 and P6 to get the closest 50 ohm reading across all the bands.

The permanent solution will be to get an authentic Phillips BRF96 transistor and insure that the instrument is stable as designed.  I couldn't have worked through this little issue without Jim's assistance.  The man is a RF design wizard.  Praise to VK5JST !

So how does this kitted marvel compare against the commercial instruments that my friends spent a lot more money on?  I've only two comparisons at present.

The first comparison was of my HF Unbalance-Unbalance 9:1 End Fed antenna.  I borrowed a MFJ analyzer some time ago.  So these readings weren't taken at the same time.  Scientifically they should have been.  Everything is reasonably close except for the 80m band.   The calibration tuning of the VK5JST was done so that it gave the best readings across all the bands.

The second comparison was done simultaneously with two other high-end analyzers.  All the SWR measurements taken in the band were reasonably close.  The magnitude readings were the closest, but for some reason the VK5JST didn't start registering reactive impedance till it was way out of the band.  The other analyzers were fairly close, but not always the same as we saw when we went out of band  (data not recorded here).

So how good is it?  Good enough for me.