Friday, December 19, 2008

The W4MMQ Legacy Balanced Antenna Tuner

By E. F. Bigbie, PE

Why a balanced antenna matching unit?

Everyone calls them tuners, including the manufacturers. But they don’t tune, they match. For this article I will call it a tuner; however it is actually an impedance matcher. It is used to cancel reactance, both inductance (+j) and capacitive (-j).

Antenna tuners are much like shovels. It takes more than one kind of shovel to perform a variety of jobs efficiently. For example, a snow shovel isn’t suitable for digging holes in hard ground. A tiling spade could be used to shovel snow, but it wouldn’t be very efficient. Similarly, no antenna tuner circuit can do every antenna-matching job extremely well.

A balanced-load tuner should be designed from the ground up for the job that it is intended to perform. This article describes a circuit that does a superb job of feeding an open-wire transmission line such as ladder line or window line. It cannot be used for unbalanced loads such as coaxial transmission line or for end-fed antennas.

Now that we have so many bands below 30 MHz, an open wire-line center-fed wire antenna systems looks even more attractive than it did when such antennas first came into popular use in the 1930s. Taking advantage of this versatile antenna system requires a box that will interface the 50 ohm unbalanced output of today’s transceivers to the highly variable impedance (Z) of the balanced feed points of multiband antennas.

Many makers of antenna tuners claim their circuits can operate into an unbalanced load or a balanced load such as ladder line. Actually most of the contemporary “matches everything, balanced or unbalanced” antenna-tuner circuits produce a semi-balanced output when used with a balanced load. Although the antenna will radiate in this situation, a semi-balanced output is like having a semi-balanced checking account. It is less than wonderful.

A look at the schematics for the contemporary “matches everything” antenna tuner circuits reveals they are usually unbalanced, high-pass-filter-characteristic, T networks with a voltage balun hooked to their unbalanced outputs. This is a compromise performance when used with a balanced load.

The imbalance in these “balanced” tuners can be easily confirmed with an RF voltmeter or RF ammeter(s). The actual current or voltage at each output terminal is progressively more imbalanced above 7 MHz. At 28 MHz, it is not uncommon to have 50% more current or voltage in one of the legs than in the other leg.

Some may ask, “Why not use the same balanced tuner design that was popular in the 1930s?” As many old-timers know, the 1930s-era balanced tuner consisted of a resonant (or near resonant) center link coupled tank circuit with moveable taps on the secondary. For each band change, the taps had to be moved and reopitmized, the total inductance changed and the tuning capacitor retuned. Changing bands was labor intensive! These tuners were seldom built in enclosures, because near-constant access to the taps and the inductor(s) was a necessity for changing frequency. It was a common practice to build these tuners on a breadboard for maximum accessibility.

In the 1950s, the E.F. Johnson Company marketed its Matchbox series of balanced antenna tuners. These tuners used the same center-link coupling arrangements as the earlier tuners but they eliminated the movable-tap arrangement by using a double differential capacitive voltage divider across the tank inductor. (A differential capacitor is the RF equivalent of a potentiometer dc-voltage divider.) This allowed the operator to increase and decrease the voltage fed to the antenna electrically, without changing taps. The Johnson circuit worked, but the Z-matching range was severely limited. Frequently, the SWR could not be reduced to a satisfactory low level.

The balanced tuner described in this article has two front-panel adjustments, a turns counter that turns two roller inductors that is connected together with two pulleys and a timing belt and a high voltage capacitor. It uses the rarely seen balanced version of the familiar unbalanced L network. Changing bands is a piece of cake with this tuner and the matching range can be made very wide using enough L and C to handle the job

The 1:1 current balun solves the capacitive imbalance problem of the 4:1 voltage baluns used in the output side of conventional tuners. The bottom line: high impedance baluns are a very likely source of grief no matter how carefully they are engineered and constructed.

All of these problems are easily avoided. The solution is simple – don’t put the balun in the highest-impedance part of the circuit. Instead, put the balun in the lowest-impedance party of the circuit (in most cases, the lowest-impedance part of the circuit is the 50 ohm coax input to the antenna tuner), and build a balanced L-network tuner for the balanced output of the low impedance balun.

So, why have we been putting the balun in the wrong part of the circuit for all these years?

Good question.

Building a no-grief 1.8 to 30 MHz 50 ohm balun is easy. You can build a Walt Maxwell W2DU type choke balun. Use 12 inches of 50 ohm Teflon coax cable and 50 ferrite beads, now available in kit form from the Wireman.

The balanced network consists of two 20 or 28 uH roller inductors connected together with a miniature timing belt and two pulleys, a counter dial and 40 to 500 pF 3.5 kV Variable capacitor, and a 50 ohm current balun on the input side.

See the pictures of the outside and inside of the balanced matching unit for the layout of the parts.

I will give all credit for what I have learned about the Balanced Matching Unit to my friend Walt Maxwell, W2DU. Walt is the author of the book Reflections and I recommend the book. It should be in every ham shack.

About the author:
Ed Bigbie received his Amateur License in 1946. He has been a member of ARRL since 1946 and QCWA since 1975. He received his First Class Phone License with Radar Endorsement in 1945 and has been an ARRL Technical Specialist since 1996. Ed was a Senior Member of the Institute of Radio Engineers (IRE) until it merged with The American Institute of Electrical Engineers (AIEE) and became the Institute of Electrical and Electronics Engineers (IEEE). Ed is currently a Senior member of IEEE and a Senior member of The Society of Broadcast Engineers (SBE). He is also a charter member of the Association of Communication Technicians (ACT). Ed has been a Registered Professional Engineer (PE) in the State of Georgia since 1965 and has authored several articles on antennas, transmission lines and matching units. He runs two IC-756PROIII, a IC-PW1 amplifier and an 811H amplifier. You can find him on 3.995 every morning from 4:00 AM to 7:00 AM Eastern time and occasioanlly on CQ100. Listen on 3.995.

Parts for a balanced antenna matching unit

The miniature timing belts and pulleys are available from Small Parts, Inc. 6901 NE Third Ave, P O Box 381737, Miami, FL 33238-1736 Phone (305) 751-0856
Air roller inductor $60.00 each $120.00
High Voltage Variable Capacitor $70.00
Turns Counter $70.00
1:1 Choke Balun Kit $28.00
Binding posts 2 ea. $6.00
Tuning knob with plate $10.00
Cabinet $75.00

Total $409.00

Saturday, November 29, 2008

RF Power Issues with the 857D

Readers of this blog know that I love this little radio. It's the first "new" HF radio I've ever owned. I've always had older tube rigs so understandably I've been quite proud to own and operate this little gem of a solid state compact radio.

It has been fun to drill down into the menus and to occasionally discover a few of it's undocumented features. One interesting thing I noticed, early on, were momentary high power "spikes" when using the rig on low power - this may not be an issue when running the rig at 100 Watts output on CW and SSB.

The ARRL and other reviews of this rig didn't show a problem, yet speaking with other users and doing numerous Google searches suggests many FT857 and FT897 transceivers also suffer from this issue. In the case of my transceiver the effect seems most noticeable in the mid HF frequency bands and is present on CW, SSB and AM. For those who use AM, the spike can be horrific and exceeds 100 Watts out when the microphone PTT is first pressed. SSB doesn't fare any better with spikes occurring on the first syllable spoken after a pause. The spikes on SSB exceed 100 Watts when the power level is set to 10 or 20 Watts.

If you are a frequent reader of this blog you know that I lost the finals in my 857D a while back when transmitting thru a solid state amplifier on 2 meters. I have since been searching for a reason as to why that failure may have occured. I'm still afraid to use the amplifier again until I am more assured as to what happened.

A Gooogle search of my problem led me to a most excellent website, Dave's Astronomy Page . I don't know Dave and have not asked his permission to republish and otherwise plagiarize his excellent research but I trust he will not be offended that what follows is actually his work.

Dave's concerns over this "power spike" were for the welfare of his linear amplifier.

Running the FT857 "barefoot" doesn't give any indications on the front panel of a problem, and indeed in many cases there isn't an issue with doubling your power for the first 5 milliseconds on transmit. This can be a serious issue when using the FT857 to drive other equipment, such as a linear amplifier. Fortunately, Dave says, his Acom 1000 linear is rated to cope with spikes of up to 100 ms duration, and these are less than that. Users of other types of amplifier may damage their equipment.

Examples of the first CW character being sent are shown below, the power output setting was 20 Watts on the 7 MHz band.

The only cure found so far is to feed a fixed voltage of between -3.0 and -3.5 Volts into the ALC socket of the FT857. Using a fixed ALC voltage still allows the power output to be set correctly from the menu in the FT857, it is also beneficial in taming the SSB spikes without causing problems to normal speech peaks.

Not all bands produce the same results, the spikes were at their worst in the mid HF bands and almost non existent on 2m. Dave says his oscilloscope is not capable of working at 432 MHz, do he doesn't comment on that band.

From experimentation, feeding what appears to be a reasonable voltage level on one band can reduce the power output on another band to near zero. This indicates that setting the voltage could be quite critical. I haven't tried the transceiver at widely differing case temperatures, nor have I tried any other transceivers to see if using the voltage my transceiver works well with also has the same effect on a another 857.

Quoting from Dave's website: The test circuit I used is shown below. Using a 9 volt alkaline battery is not a good choice as the maximum voltage required is only -4 Volts. The output (tip and sleeve) are wired to a 3.5mm jack plug, which is inserted into the ALC input socket on the FT857D. The 1.5 K Ohm series resistor avoids shorting the battery out when inserting the jack plug into the transceiver. For longer term use 3 or 4 AA sized alkaline batteries would be a more sensible source of the voltage. Note the FT857D manual gives an ALC input range between 0 and -4 Volts in the specifications. The optimum ALC voltage feed on my transceiver was -3.1 Volts, this level still allowed full output on 2 meters. A setting of -3.6 volts worked well on HF and 6 metres, but killed the output completely on 2 meters.

A more elegant solution is to generate a negative voltage by the use of an NE555, as shown below (note the 47 uF capacitor across the zener was incorrectly shown in an earlier diagram). A 13.8 volt supply is available from the "linear" socket on the FT857. Be careful when wiring a mini DIN plug as they are quite small and tricky to solder without creating a short. Covering the rear of the pins with hot melt glue after soldering the wires helps keep everything secure and short free - the plastic glue can be peeled off for re-work without difficulty. The photo of the completed box also has a phono socket connected to the TX ground pin of the mini DIN plug to give a feed to my Acom linear.

Dave says he contacted Yaesu for their view on this problem and they apparently have a key click modification that is applied to the current FT857D series, from board 07 onwards.

Anyone wishing to modify their FT857 using the modification below does so at their own risk. While the modification was supplied by Yaesu, it doesn't make it easy to implement. The modification can be downloaded here .

The central part of the underside of the main board in an FT857D is shown below (not modified).

Tuesday, November 11, 2008

How to decode Yaesu serial numbers

If you need to know exactly when your Yaesu radio was manufactured here is the way to decode Yaesu serial numbers:

Position 1 = year made

Position 2 = month made

C = January
D = February
E = March
F = April
G = May
H = June
I = July
J = August
K = September
L = October
M = November
N = December

Pos 3 &/or 4 = lot number(s)

Pos 5<-8 sequence in that lot

Position 1: last digit of the year in which the radio was produced

Position 2: Determine the position of the letter in the alphabet (C=3,D=4,E=5, etc) and then subtract 2. The result gives the month in which the radio was manufactured. Note that the letters will range from C (January) throughN (December). Thus, a radio with a serial number of 5H221234 would have been manufactured in 2005 (5), during the month of June (H=8, 8 minus 2 = 6, June is the 6th month), was part of lot 22, and was the 1234th radio in that lot.

Saturday, August 30, 2008

Monitoring the Gulf Coast Hurricane Net

One of the more interesting aspects of amateur radio for me has always been monitoring the Gulf Coast Hurricane Net when there was a storm threatening the Gulf Coast area. As of Saturday evening, the net is active on the following frequencies:

Central Gulf Coast Hurricane Net
Daytime Tactical: 7.285 Mhz
Nighttime Tactical: 3.873 Mhz

Daytime Health and Welfare: 7.290 Mhz
Nighttime Health and Welfare: 3.935 Mhz

Sunday, August 10, 2008

Tracking down the elusive electrical noise in my home

I have finally found the annoying “mystery noise” I’ve been hearing on HF (mostly 75 meters) today. I first begin to hear the noise after I moved my Yaseu 857D from my outdoor (not air conditioned) ham shack to the comforts of a spare room in my home. While it is great to have the space (and air conditioning) I immediately began to hear a crescendo of noises that sounded like “birdies” or raspy “carrier like” noises. In one case the noise appeared every 100 KHz pretty much from the broadcast band to 10 meters!

I first had to determine if the noises were radiated signals or were, instead, internal “birdies” sometimes heard from receivers of poor design The way I did this was to switch the 857 to dummy load and verify I could still hear the noise – I still heard it even though it was somewhat weak.

The next step was to determine if it was an internal “birdie” from the 857 or a radiated signal. By disconnecting the HF antenna all of the noise disappeared so I was pretty sure it was not a birdie.

At this point I became convinced the source of the noise was something electrical inside my home. I powered up the 857 from a couple of AGM batteries and tuned the VFO to a spot where I could hear the noise quite loud. I then went to my power switchbox and tripped the main breaker.

What do you know – the noise went away! In fact, I’ve never heard the band so quiet!

After I turned the main breaker back on it was just a matter of tripping the individual breakers until I found the offending circuit. Turns out the noisy circuit was one feeding one of those “touch lamps” – the kind where you touch the metal lamp base and the bulb steps thru three levels of brightness. Those things are awful noise generators! I unplugged the lamp, put the breaker back on and listened to verify the noise was gone. Thankfully it was. But, then I heard yet a different noise which I quickly tracked down to a horizontal sweep signal from the TV upstairs.

That was easy to fix -- just turn off the TV.

But wait – I was hearing yet another raspy noise on the frequency. This one was a little harder to find but it was definitely man made inasmuch as it appeared on even frequency multiples across the band. I suspected the source to be a switching power supply, perhaps my UPS or maybe even the laptop computer itself. It took a while to pin it down but when I unplugged the laptop charger the noise went away – and resumed when the charger was again connected to the computer. It didn’t matter which end was unplugged, the AC supply end or the DC cable at the computer.

So, now I am finally happy. I have found the offending noise and those I can’t eliminate at least I know what to turn off should I want to listen to the same spot on the dial where these local oscillations are occurring.

In most cases it would not even matter but this morning the touch lamp was just tearing up the Georgia Cracker Net on 3.995 MHz and that is what got me started. I would have probably just written it off as a distant carrier on the band but because I was able to listen to my friend Lyndy’s Brannon’s station via his streaming audio setup I knew it was not affecting the entire net – It had to be something local.

Friday, August 8, 2008

Blown VHF Finals in a Yaseu 857D

I am officially a member of the infamous "Blown Finals Club" of Yaesu 857 and 817 owners.
Today, after a $230.00 repair bill and FedEx shipping both ways I have my radio back from the Yaesu Service Center in California and other than a huge void in my wallet, life is good once again.

So how did this happen?

I'll never know what caused the final VHF transistor and several capacitors to suddenly fail but at the time I was using the radio to transmit thru a 45 watt in - 170 watt out linear amplifier. The antenna is a 2-meter 5/8 wave ground plane that I've been using for months. Likewise, the 857 has been transmitting thru this amplifier for many months though the amplifier is almost always off.

This time I was transmitting about 45 watts FM on 146.52 into the amplifier (now switched on) and carried on a simplex QSO for a few minutes. We then switched to 144.20 SSB and continued the QSO briefly. I was still using the amplifier, only this time the rocker switch was switched to SSB. Having an unsuccessful QSO I switched the Yaesu 857 back to FM and changed frequency back to 146.52. I keyed the mike and became aware that I was not transmitting. I noticed the amplifier was still switched to SSB so I flipped the switch on the amp back to FM. Still no transmit power from the 857.

I tried several tests in various configurations, first without the amplifier. I found that I was actually able to be heard on the local repeater although I was reported quite noisy. This led me to conclude that I was transmitting with at least the driver stage of the transmitter. I switched to a good dummy load and then monitored my 2-meter transmissions on a VX5R handheld I had in the shack. Everything sounded well -- good audio etc.

At this point it became painfully obvious I had a blown PA transistor. By the way, the radio continued to function properly on the HF frequencies. Only 2-meters and above were affected.

I carefully packaged the radio in its original box and with a humble letter enclosed I sent it on its way to the Yaesu hospital in California.

Saturday, May 24, 2008

First Entry

This is the first entry on my Blog