RaDAR – Feedback RaDAR Challenge 2017-07-15

As far as ZS was concerned different times were planned between the various RaDAR teams but I chose mine to coincide with the local QRP contest with the hope of being able to make sufficient contacts to make a few transistions on foot. Because there were different time frames involved I thought I would take young Eduan, my grandson, along for an outing to look out for active RaDAR operators – we would be RaDAR chasers!

I have an old military HF radio, the popular B25, and I made a plan to strap it to a frame from a “child carrier” which I bought right back in the days of SiaS (“Shack in a Sack”) and that’s a long way back! It worked PERFECTLY for the purpose and has now become a permanent fixture for RaDAR ops! I’ll make a plan with the DC / Battery wiring that it also be permanently mounted. This is so comfortable you could easily run with it! The cross strap is there for additional safety but not really required.

It was supposed to be the middle of winter but as you can see still hot but with a cool breeze. At the time of writing, today is Monday, two days later and the first real cold for the winter has finally arrived.

Eduan and I took a kilometer walk and did a deployment using the B25 and 7 A/Hr SLAB. The antenna was a fixed tuned 40m end fed with a short coax feed. Eduan did the antenna deployment using a nylon pull rope and rock to get it over a tree branch then hooking the centre of the end fed to the rope and pulling it up into the tree. We made three QSO’s before going back home. He couldn’t use the radio though and I had to do all the talking. A special QSO was with Pieter V51PJ in Namibia, an active RaDAR supporter. That proves too the strange 40m conditions at present, usually a NVIS band but far from being a NVIS band now!

Once we got back home I got ready to move out using my RaDAR pack and painter’s pole RaDAR mast. I walked down to the river at 14:00 local and set up the link dipole and FT-817ND using it’s new 7 A/Hr SLAB. Conditions were really not good. 40m has been skip for a few weeks / months now and the winter was still hot, like mild summer days. Nature is in turmoil!

Here I sat listening, calling and the only contacts were with Sid ZS6AYC also doing RaDAR and Herman operating the club RaDAR station ZS5HAC. ZS6 was skip to me … and seems other divisions too – I heard nothing. On 7.020 I heard one or two short transmissions on CW and that was all. Not even the AWA CW net was active – NOTHING!

So I waited for Sid to move again that I could be there for his last transition. They were on foot this time round, him and his wife Adele ZS5APT. What a team!!!

After that I realized it was futile to continue. I wasn’t even sure if anyone was actually even active in the SARL QRP contest???

I went home for coffee and milk tart totally disallusioned with amateur radio. I’d hoped for some real winter conditions in support of Julian who braved the elements, snow and ice rain in April. It was just another summer’s day for me …..

I changed my actual RaDAR ops times to include the chase having done it in real RaDAR fashion and in so doing making my total 6 and not just 3 for a four hour challenge with two transistions a two kilometer walk between them.

 

73 de Eddie ZS6BNE

 

 

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RaDAR- The HB1A PA in pictures

I’ve battled with the output power of my HB1A from the start and there is not much info on the Net. I’ve accumulated some info from various sources to help me understand this relatively simple but “technically sophisticated” RF power amplifier.

The PA is as simple as this!

During my tests I accidentally exceeded the drive to the transistor by removing R35 (I later put in a 1/4 W 47 ohm resistor in place of the tiny surface mounted resistor) and burnt out the 10uH inductor L6.. It needs to be replaced and I was thinking of winding a 10uH inductor on a T50-6 toroid and putting that in it’s place. The toroid winding info would thus be:

I wanted to understand the real purpose of RF Choke L6 and found this discription in an ebook “Crystal sets to SSB”.

That says a lot and I would then regard that component as being a nescessity especially with regards to the problem I have!

The upside of choosing correct inductor (L) is that you get higher efficiency of the transmitter and your transistor will run much cooler. The peak collector current will be larger too. Peak collector current is larger because while ON, the transistor builds extra current through the small inductor (L). This current is dumped into the pi-filter (during the OFF phase) and must be replenished during the next ON period. The result is that the filter sees a larger swing, which translates to slightly larger output RF power. So you can see that the extra voltage and current that the transistor encounters is not wasted.

Just before the PA circuitry we have the HB1A’s DDS (Oscillator) and buffer circuitry and this explanation says a lot.

So I would assume that the HB1A’s final PA is what is referred to as a Class C amplifier.

From the Net …. “Every transistor has what is referred to as a “threshold”. This is the voltage at which the transistor begins to conduct. By it’s nature, a radio signal consists of a train of waves. A wave consists of varying voltages and/or currents. A sine wave, as it pertains to radio, typically has a positive portion (half of the wave consists of positive voltages) and a negative portion (the other half consists of negative voltages). The simplest and most common transistors (BJTs or bi-polar junction transistors) have a threshold of .7V. This means that when a wave is input into a single BJT transistor in the simplest possible amplifier, the transistor only produces output when the input wave is at .7V or higher. Obviously this means that that the rest of the wave (the entire negative portion and some of the positive portion) is missing from the output signal. The amplifier is conducting less than 180 deg. which means most of the original signal is missing. This is a classic example of nonlinearity. We put in a sine wave and we get only part of it back. This chopped up output wave is said to be “distorted”. This describes “class C” amplifiers. Even though the output is distorted, class C amplifiers are still very useful ….. “.

From Wikibooks :

Class C amplifiers conduct less than 50% of the input signal, typically only conducting during input peaks. Distortion is high, but high efficiencies (up to 90%) are possible. Some applications can tolerate the distortion. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to switch the amplifying device on during peaks, and the complete waveform is recreated by a tuned circuit.

The Class C amp has two modes of operation: tuned, and untuned. When the proper load (e.g., a pure LC filter) is used, two things happen. The first is that the output’s bias level is “clamped”, so that the output variation is centered at one-half of the supply voltage. This is why tuned operation is sometimes called a “clamper”. This action of elevating bias level allows the waveform to be restored to its proper shape, allowing a complete waveform to be re-established despite having only a one-polarity supply. This is directly related to the second phenomenon: the waveform on the center frequency becomes much less distorted. The distortion that is present is dependent upon the bandwidth of the tuned load, with the center frequency seeing very little distortion, but greater attenuation the farther from the tuned frequency that the signal gets.

The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be extracted by the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter.

 

 

There are a lot of similarities with this circuit and the actual HB1A’s transmitter circuit.

The HB1A’s output filter circuitry is as follows:

So back to the workbench. Building in a rugged 10uH RF Choke and maybe even replacing the RF transistor. Then all should be good to go!

As simple as that …………. or is it?

73 de Eddie ZS6BNE

 

 

RaDAR – A companion for the YouKits HB1A

The YouKits HB1A comes in various flavours also distributed as two band units by TenTec. I have the 3 band YouKits version, 40m / 30m and 20m. I needed to build a trail friendly, finely tuneable antenna (for maximum throughput at QRPp levels) to cover these three bands, a companion for the little rig. The best option in my opinion was a tuneable end fed.

I built the radiator much like a linked dipole, a half wave length for 20m, [link] a half wave for 30m and [link] a half wave for 40m.

From a previous post – The calculated lengths for the radiators are: (Midband frequencies)

7.1 MHz – 20.6 meters

10.125 MHz – 14.4 meters

14.175 MHz – 10.3 meters

The tuner configuration I built falls in with this scenario:

I used 3 turns on the primary and 24 turns on the secondary wound on a T80-2 toroid. That gives a 1:8 turns ratio and is seen as a 1:64 impedance transformer. 50 ohms on the primary and 50 * 64 = 3200 ohms on the secondary. That’s close to an end fed’s feedpoint impedance but not always exactly. If it were 5000 ohms then we’d see an impedance on the primary of 78 ohms which would be a mismatch but not that much. A little unremoveable SWR. As long as it’s under 2:1 but in most cases just about 1.1:1 is achievable by tuning the capacitor C.

I built the tuner into a tiny plastic box using a piece of PC board to mount the toroid and to make the connections. This makes it reasonably rugged. The capacitor is fixed in place using contact adhesive.

The radiator wire is permanently soldered into place and I also have a randomly chosen, around 1 meter in length, counterpoise wire that just lies on the ground. It’s just a piece of wire which makes me feel better about the “return path”. This sometimes acts as a “guying” or “stress relief” system anchoring the one side of the antenna on the ground.

The fixed and, soldered into place, coax cable and BNC connector to the rig also add to the ruggedness of the tuner.

For lack of a better control knob – a temporary workaround:

Using the FT-897d running off a battery in the back yard, I did tests on all three bands. On 40m and 30m I was able to get zero indication of SWR on the rig’s display throughout each band. It was more difficult to do that on 20m but there was a dip in SWR at almost full clockwise adjustment of the capacitor. Anything more than 10W saw spikes in the SWR which was probably caused by flashovers on the capacitor so this is really a QRP ONLY tuner!

Using the mini RC-Calculator 1.2 program it would appear that the secondary’s inductance is 3.16 uH.

 

Using the same calculator the capacitance values required for the parallel tuned circuit for resonance at a particular frequency is as follows:

 

 

73 de Eddie ZS6BNE

 

 

RaDAR – Fishing for satellites

Sunday morning and the sun has just risen, I’m awake and in time. The family still sleeping and I silently get dressed without disturbing them. I have an apointment and minutes count. I’d done my planning the evening before – the mission was clear.

I needed to be down by the river and ready by 07:25 CAT for SO-50, an awesome FM satellite. I’d made my intentions known through the local WhatsApp groups the night before. I needed to carry my Arrow yagi with strapped on battery seeing my TH-D7’s nicad battery has been removed – now useless being many years old. There is a penlight battary pack attached but essentially only there for reception and off the air setup.

It was a cool morning, the middle of winter in South Africa but not really that cold. Lots of dew on the ground but it wasn’t frozen. I had no gloves on and a light top just to keep the wind at bay.

The TH-D7A, my MP3 recorder,  mic, earphone and compass were all packed into my lightweight backpack. When I got to the deployment point I hooked everything up …. except the battery. The internal penight cells gave me a false sense of security but they failed on transmit …. then I remembered the DC cable for the Rossi battery. I hooked that up too, set the required frequencies and waited for the satellite to come over the horizon.

 

It wasn’t necessary to determine direction using my magnetic compass as I already knew where true north was, nevertheless I imagined the path the satellite would take through the sky. My smartphone was lying on the ground aligned to true north and I watched as the ISS Detector app notified me of the satellite’s position.

At a relatively low elevation I heard Tom, ZS1TA calling. It was an easy QSO and we exchanged grids. Christi ZS4CGR was portable nearby, both of us in KG34, Christi in Zeerust and I alongside the Molopo river a 100 km away. That was an easy QSO too. Only the three of us up this early Sunday morning.

It was awesome to work a satellite this way proving it’s effectiveness operating from the field in true RaDAR style.

I went back home for coffee. A celebration of another successful RaDAR excursion.

Next weekend will see many radio hams throughout the world practicing RaDAR during the RaDAR Challenge. Each gets to plan his / her own four hour session within a twenty four hour period. I will be mostly HF QRP using SSB or CW and moving a kilometer on foot after every five QSO’s. Fortunately the QRP contest takes place in the afternoon and I have chosen my RaDAR Challenge times to coincide with it hoping to find sufficient HF activity.

Good luck to all for next weekend!!!

73 de Eddie ZS6BNE

 

RaDAR – Another interesting L-Match designer

I found this rather interesting website with online calculator. Certainly there to make life easier for the end fed antenna enthusiast. It also follows the same lines of Pierre ZS6A’s spreadsheet mentioned before in other topics and should be followed.

I have chosen these three bands typically because it’s the bands my HB1A can transmit on. My HB1A is constantly a work in progress since I had to repair it myself and see chance in modifying it. See https://zs6bne.wordpress.com/2017/05/19/youkits-hb1a-replacing-the-lcd-module/

It makes sense to build a 3 band end fed tuner into the rig as that is typically the antenna I would use with it! So …… the next project is ….. and now to figure out the best way to do that with minimum modification.

See http://www.daycounter.com/Calculators/L-Matching-Network-Calculator.phtml

Note the Q Value stays constant …..The Q (bandwidth) of a filter is fixed for an L matching network, because it is set by the ratio of the source and load resistance which are typically fixed values.  For a variable Q use a Pi or T matching network.

Some examples executed there:

40m – 7.1 MHz

From Pierre’s spreadsheet

 

30m – 10.125 MHz

20m – 14.175 MHz

That basically comes down to this table of values ……

Over complicated inductance switching maybe ?

Or simply ……….. a 1:64 impedance transformer and parallel tuned circuit on the secondary.

See https://zs6bne.wordpress.com/2017/06/30/radar-the-end-fed-tuner-dilemma/

A 10 uH inductor wound on a T80-2 toroid will need 43 turns (103 cm of wire) and the following capacitance values / band.

40m – 50 pF

30m – 25 pF

20m – 13 pF

Or using the values from the above link :

A 2.8 uH inductor wound on a T50-2 toroid will need 24 turns (60 cm of wire) and the following capacitance values / band.

40m – 178 pF

30m – 78 pF

20m – 40 pF

So, which is best ???

 

As calculated using Pierre’s spreadsheet, I will make the radiator like one leg of a linked dipole.

The lengths of the radiators are:

7.1 MHz – 20.6 meters

10.125 MHz – 14.4 meters

14.175 MHz – 10.3 meters

I was thinking of using the hole usually used for the BNC female adapter as a place to mount a varicon capacitor! Hopefully it will fit there!!!

The T80-2 could be fixed to the back cover (Usually where one would foolishly have penlight battery holders) and also holes drilled and fitted with female banana plug sockets for the radiator (end fed antenna) and a short counterpoise wire that can lay on the ground.

The plan is coming together …… I think!

 

73 de Eddie ZS6BNE

RaDAR – 160m end fed half wave antenna building instructions

Antenna building instructions / article for Radio ZS

This is about the easiest way to build a 160m antenna. It’s up to you to get it as high as possible. An inverted L shape is good, even vertical if you can but if you have no other choice get it off the ground if only a few meters high. You’ll still have some NVIS (Near Vertical Incidence Skywave) fun with really good signals – after dark!

Firstly, you’ll need a 100 meter length of copper wire that will be strong enough and good enough and the ideal for this is a 100m roll of 1.5mm electrical house wiring wire from your nearest electrical outlet. It will cost around R200.00 If you want everyone to see your wire, choose red or green / yellow but if not, choose black.

Unroll the wire making sure there are no kinks, loops or tangles. Take your time and enjoy the sunlight! Once unrolled, measure off 20 meters and cut the wire leaving hopefully a length of 80 meters behind. The 80 meter length of wire will be your radiator and the 20 meter length will be your “counterpoise”. You’ll need the counterpoise otherwise RF will be looking for a path to ground in all the places where it shouldn’t!

Good news, you’ll only need a short piece of coax from your rig to the magic box. Inside the magic box you’ll have a toroid with a few turns of wire around it. You’ll also need to find a variable capacitor which may be the most difficult obstacle. Find one that is around 200 pF or more. The vane spacing is usually not critical unless you want to use a few hundred Watts where you’ll need wider spacing otherwise you may encounter arcing between the plates.

We are going to feed the antenna from the side, not in the middle like a dipole or inverted vee. That means we need to match the rigs 50 ohms to the antenna which is 2750 ohms – or more. That’s easy of course using a low pass filter and it will supress some unwanted harmonics too!

My friend Pierre ZS6A, drew up an interesting table of suitable inductance values for various bands and feedpoint impedances.

From the table we see we need an inductance of around 32uH if the feedpoint impedance is around 2750 ohms. If it’s higher we’ll need a little more inductance but it’s impossible to know exactly what the feedpoint is. An inductance of 32 uH worked well for me and a 200 pF variable capacitor was sufficient too.

You will need a suitable toroid. I had a T157-2 in my parts box and used that. This is a red toroid suitable for HF frequencies. An excellent website on determining what toroids you have available in your box can be found at http://toroids.info/

There is also an online winding calculator which you can use to see how many turns you need to make on your toroid to get the required inductance.

There is an excellent program you can download from the Internet to your PC to make calculations on your toroid inductances. It can be downloaded from http://www.electronicecircuits.com/download/software/mini%20Ring-core%20calculator%20program.zip

This program gives you additional information like the affects of power on the toroid. For QRP 5 Watts, the voltage on the toroid would be V = SQRT (P x Z) and with a feedpoint impedance of say 3000 ohms, 5W would look like this. I’d loose 300 mW through the toroid which isn’t that bad at all!

At 100W the T157-2 would start to take a little strain as far as heating is concerned but that’s quite normal! An odd 7.4 Watts are lost through the toroid. Tradeoff …. lots of coax feeding the wire in the middle like in a dipole with the logistical complexities of a very large dipole or the end fed. The end fed offers many alternative options.

Note the 20 meter length of wire is connected to the earth side of the capacitor – make that the rotor then the control shaft is also at earth potential so there is no chance of getting a RF burn while tuning. Of course that’s the same potential of the rig via the coax braide.

          

Tune for minimum SWR. It’s pretty easy to get 1.1:1 with this setup but don’t leave the counterpoise unconnected! The counterpoise can simply lie on the ground.

Note, I built mine into an old modified MFJ945e antenna tuner with built in cross needle SWR meter. It can be much simpler, the toroid and capacitor in a suitable little plastic box.

At the far end (80 meters away) , terminate the wire on an electric fence insulator and tie down with a nylon rope. The ends are at a high voltage low current point.

Enjoy 160m, there are quite a few ham radio operators in ZS who would love to have a QSO with you!

 

73 de Eddie ZS6BNE