Portable preamps for 6m 2m and 70cm – part 3

Hi all,

This is continuing on from Portable preamps for 6m, 2m and 70cm – Part 2.

I have updated some board designs for both the preamp and the sequencer.

Here’s a look at the preamp board:

Preamplifier board

And here is the sequencer/dual bias tee:

Preamplifier sequencer and dual bias tee

As stated in the previous post, this bias tee is designed to operate both of the FT-817 and FT-897 antenna ports. The caps exposed to full brunt RF are much more expensive than the X7R caps that can be found on ebay, but this is part of the cost and convenience of having something that will bias the coax and allow 100 watts SSB to pass. The caps are rated to 5 or so amps, so this imposes a SWR limit. It is on the low impedance side, but of course the actual current varies along the cable in the presence of SWR due to the standing waves. The FT-897 will reduce power in the presence of high SWR, but this board could have its ratings exceeded if driven with 3 or higher SWR at full power. Of course, I have also considered some margin for the inductors that will face high RF power as well.

Away from the high stakes, I can use cheaper (much cheaper) caps. The relay is also a little expensive, but if you wanted to order 1000 boards plus components, I’m sure that just about all the components will get much cheaper!

There is no need to actually use this board for two bias tees, by not installing two inductors one capacitor and the two BNC jacks associated with one channel. All the other components remain required.

The 4401s are a few cents, along with the resistors. Both boards are designed for a BNC through hole jack. It is still possible to run alternative connectors, but these would need to be case mounted – eg a UHF or N connector, with a short run to the holes on the board for where the BNC jack would go. The planned BNC jacks also provide for case mounting, so the board will be held to the case through the jacks.

The preamp board has less scope to use cheap components. The relays selected there must be rated for the RF current going through – the AXICOM HF3’s are rated to 60watts continuous RF power and 2 amps. Most of the caps and inductors again must face high power RF.

I’m in the process of ordering parts and getting some boards printed to build this thing and give it a spin. I also look forward to increasing my contacts next John Moyle field day.

73 de Wayne VK3WAM

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Portable preamps for 6m, 2m and 70cm – Part 2

Hi all,

This is a continuation of a post at: Portable preamps for 6/2/70 part 1.

Part 1 showed a schematic for the preamp component. This would live on the antenna, near the feedpoint. Given the bias tee, it will only need to be fed by coax. It does not need any control lines or separate DC cables going up there.

Here is a picture of the PCB design:

Preamplifier board

I’ve got this under 2 inches by 1.5 inches, so I am happy with the size. I have two BNC connectors on board, these can be obtained from rfsupplier. I used microwave design techniques on this board with as much as possible of the RF being microstrip with 50 ohms characteristic impedance. Of course, with this approach, there is no using vias for anything apart from ground connections. In line with Minicircuits recommendations, the PGA-103 is surrounded with grounding vias. I also ensured that there were grounding vias near each of the relay RF inputs and outputs.

Sequencer design

The sequencer needs to do several things:

  • Provide the Vdd supply for the preamp and put this on the coax
  • Only put Vdd on the coax when the radio is Rxing – otherwise the preamp will be exposed to RF far beyond spec, and magic smoke will not be far behind. As the preamp uses non latching relays, any removal of DC from the coax will cause the relays of the preamp to switch to bypass, allowing TX RF to safely pass.
  • Allow for a delay between the radio commencing TX and high power RF going out the cable. Modern full mode rigs have TX inhibiter inputs, and the FT-817, 57 and 97 are no exceptions. The microprocessor can respond in a few microseconds, but relays need a few milliseconds to switch. The plan is to allow 25 milliseconds to be safe.
  • Also provide for a short delay when TX stops before switching back to RX. This would be about 10ms.
  • The local bias tee to put the DC on the cable needs to withstand 100 watts ssb. This will mean this bias tee will not need any relays to switch the RF. Of course there are relays in the preamp at the antenna end of the coax, where they are needed.

Microchip manufacture the PIC range of controllers. The task needed here is basically a timer and a state machine to deal with the various scenarios. Others who have built sequencers have used PIC controllers. In a previous life, I wrote real time control systems on PCs. The requirements here are far more simple! Looking at the range, the entry level PIC10 series will do the job. While not in the data sheet for the device, the Microchip web site also details that this device has current source/sink capability of 25mA. This is quite nice, and with a 5V supply, should give lots of drive and sink options.

On the FT-817 and 97 there is an ACC port. This has:

  • TX GND (a sink when the radio wants to TX, otherwise floating – i.e. open collector),
  • TX INH signal. This input feeds the base of a BJT through a 47k resistor in the radio. The transistor needs to sink 8V through a 3.3k resistor to prevent TX. This is 2.4mA. We need to provide 1/Beta of this current, and Beta should be 40+, so only 53uA or so is needed.
  • A 13.8V (or actually whatever the voltage that is feeding the radio is – so on a FT-817 this could be 9V for instance. There is a 10ohm 1/10th watt resistor on the radio in series with this source, so this source is practically limited to about 80mA. We will not want to power things from this, but we can present high impedance to it and use it as a signal to indicate the presence of the radio.
  • A GND

Here’s a look at the circuit:

Preamp sequencer for 6/2/70

EDIT: This diagram is updated to correct a small drawing bug.

I thought about whether I would use a transistor to switch the 5V supply to the bias tee and onto the coax, but there could be 300 to 400mA being pulled when on, and this is a bit to sink when it needs to be off. Instead, I introduced a relay to switch the supply voltage to the bias tee.

There are actually two bias tees. One for 6m and the other a compromise between 2 and 70. Both the FT-897 and the FT-817 have two antenna connections. The 897 is not selectable, one is HF/6m and the other is 2/70. I have largely gone with this configuration, however the 6m optimised tee could drive 2m. The FT-817 is fully switch-able, so either of its antenna connections can be used for any band. I have selected bias tee components that can be switched, so one of these circuits could have the left optimised for 2, the other for 70, or have both bias tees the same configuration. Again, I selected bias tee components to sink the fraction of power that will go through the tee from TX power at 100 w ssb, CW or 50w PSK. This is actually one of the key reasons for two inductors on each tee. One is a ferrite coil, the other is a chip inductor. Both are from Coilcraft and the part numbers are going to be quite specific. It will not be possible to use any inductor of a similar inductance value, because the actual inductance at the target frequency can be quite different to expectations. There is also the need to actually sink some of the power that comes through here as inductors are not known for having high Q – Q effectively being a ratio between inductor resistance and reactance.

The relay needs around 12mA to drive the coil. I am using two 4401’s with the coil on the collector of the first. Q1 has a PIC output on the base. If this is taken high, then Q1 will conduct. 2mA will be drawn from GP0 on the PIC, well within the 25mA limit. Q2 has the radio 13.8V output on its base. The resistor is sized again to provide for a 2mA draw. If the radio is connected, then Q2 will conduct, otherwise not. This setup is effectively and AND operation. Both the PIC output and the radio 13.8V signal (which just needs to be any meaningful non-zero voltage) are required for the coil to switch from NC to NO and provide the bias voltage onto the bias tees. No radio connected to this sequencer, then no Q2 conducting, and therefore no DC bias on the coax. This results in the preamps at the other end being safely deactivated and bypassed.

GP3 is an input only pin and it is connected to TX GND. This is an open collector pin, acting as a current sink when activated, with very low resistance. About 1mA will be pulled out of the GP3 chip when low. The PIC has TTL inputs, and also provides weak pull-ups, so no external pull-up resistor between TX GND and Vdd is required.

GP1 has a 1k resistor, which is effectively in series with the 47k resister in the radio. This should provide about 110uA of drive, again a tiny fraction of the 25mA PIC capability, and about double what is needed by the radio for TX INH. The 1k resistor is there mainly to protect the PIC in-case of an interface cable short – only a little less than 5mA would flow in case of a short.

The circuit has a number of 1nF capacitors. For the ones not associated with the bias tee, I can use some cheap X7R dialectic caps, but little compromise can be made with the 1nF cap in the bias tee. ATCeramics make some good high RF power rated caps for that job, I can use cheaper Ebay caps for the rest.

Finally, the code in the pic, which I will need to write, basically needs to implement the state machine. This code should be pretty straight-forward.

Next step is to design the board, and then it will be time to get some boards made. I have also designed a FT-817 to phone interface for PSK – see post here – and so I will get a batch job done with a few of each of these boards.

73 de Wayne VK3WAM

Part 3 can be found here.

Portable preamps for 6m, 2m and 70cm – Part 1

Hi all,

One project that I have been working for a little while is the design and eventual build of preamps for the 6m / 2m / 70cm bands.

My requirements are:

  • Relay switching for TX, by the presence of DC supply
  • Preamp activated by DC power, otherwise off and bypassed
  • 100 watt TX capability, but only at 50% duty cycle
  • DC supply for the preamps to be on the coax, so bias tees will be needed
  • Said bias tees must also work in the presence of 100w TX power

Why preamps?

The question needs to be asked. These bands are often quite low noise, certainly on many SOTA (Summits on the Air) mountain tops. Most of my SOTA activations have been HF, however there have been a few that have been on 2m. During the John Moyle Field Day contest this year, I operated from Mt Torbreck, VK3/VN-001. While I got a good score, there were a few QSO’s going missing who could hear me, but I could not hear them. Some of them were using more power than me. Some have reported that they hear much better with preamps. So, there seems to be enough of a reason to do it.

DC power supply from the coax

It takes some design work, but clearly it is going to be convenient to have DC power come down the same cable as the coax from the radio. Near the radio, I will need a bias tee to put the DC on the cable, and it will only do this when not TXing. When it gets to the preamp, there are three possible states:

  1. The radio is TXing and there could be lots of RF energy on the coax, no DC
  2. The radio is RXing and there is DC on the coax
  3. The radio is RXing and there is no DC on the coax

State 1 needs to ensure that there is not large amounts of energy that gets on the Vcc rail in the preamp. An inductor blocks (in part) AC, including RF. It does it through reactance, and because its Q is not infinite, some resistance as well. Unless the Q is very low, it is by far through reactance.

State 2 needs to get the DC to the Vcc rail of the preamp. This will flow happily through the inductor, but the inductor does have some DC resistance – so there is going to be a voltage drop. The DC should not continue further down the coax towards the antenna, so a DC blocking cap is needed – this needs to have very low effective series resistance and very low capacitive reactance at the RF frequency. It also needs to be able to handle the RF current during State 1.

State 3 has no special circuit requirements for state 3 as no DC power is on the coax, and the inductor is going to block the RX RF energy from going up into the preamp, instead it needs to go to the radio where hopefully it results in received intelligence 🙂

The impedance of the inductor needs to be high, much higher than 50 ohms at the RF frequency. It needs to have low DC resistance. It also needs to take into account the fact that inductors are imperfect beasts at RF frequencies – they have resonances and act like capacitors above the resonant frequency. I was able to use some design formulae from RF Circuit Design by Chris Bowick. These give a whole range of impedance matching values derived from S parameters. It can also be used to find actual inductance and capacitance values for a given component at a given frequency. The nominal inductance of an inductor is not a fixed value, but changes with frequency. Different inductors change in different ways, based on their construction.

Getting the right inductors has been a major challenge. Even if I have an insertion loss of only 0.2db, there is still about 3 watts getting into the preamp circuit if there is 100 watts of RF outside, and some of this will have to be dissipated by the inductor’s resistance. Also, some of these inductors are quite large, but if they are too big, their resonances will be at far too low a frequency to be useful for the targeted 6m, 2m and 70cm bands.

In the end, to deal with power dissipation requirements, I used a specific bias tee inductor and some chip inductors. If I used just one inductor, I would lose a little too much DC across the tee (x2 of course for the fact that I need to put this DC on the coax back near the radio!)

Ripple on the Vcc rail

So now, I have got my DC when I want it, and are not likely to release magic smoke when at 100 watts 50% duty cycle modes. I still have some ripple on the supply, and I need a bypass cap to sink it to earth. The inductors suck up most of the heat dissipation. If I use a single cap, this leaves a lower frequency pattern, so I have found that having two caps, one around 1nF to 5nF based on the band (higher band, lower cap), and a second 10uF cap seems to get the ripple down to less than a millivolt. In (a different) RF Circuit Design by Richard Chi Hsi Li, the point is made than rather than having a capacitor forest of low to high values, a single cap should be selected around self resonant frequency (SRF). This exact frequency is going to vary, and the cap cannot be ordered with some precise, eg 2.2235nF value, so I target a little below SRF for the small cap. Some simulation tests show that these caps are only dissipating a few milliwatts, so it is looking good.

Relays

The next thing to discuss is getting the amplifier that I am going to use out of the way during TX, or I will release the magic smoke. I have found a suitable RF surface mount relay that looks suitable for the power rating required from Axicom. It has a continuous 60 watt rating, so 100 watts SSB or even 100 watts CW should be a piece of cake. Lets not try 100 watts PSK though! I will need two of these relays, one each side of the amplifier. Normally closed is no DC voltage on the coax, state 2 and 3. This would bypass the amplifier with the signal going directly from one relay to the next. Normally open will connect the amplifier.

One issue is that using the same relay, the RF paths have to cross. If I use a two sided board with a ground plane with the RF signal going on microstrip, I cannot cross the signals on the board. I am going to have to have a short jumper on the board for one of the signals – I have chosen the normally open (preamped signal).

The amp

From the Antenna side relay normally open the RF comes into the amplifier. Now, I could just use a JFET or MOSFET, or even a MMIC, but Minicircuits have a device called PGA-103+ which should make this part real easy. It has three (well four, but ground shares two) pads. One for RF in, one (two) for ground, and one for RF out and DC bias. This device works on either 3V or 5V. The device needs a DC blocker cap on the input, and one on the output. The bias voltage needs to get in there somehow, and so this calls for another bias tee on the output pin, before the output DC blocker cap. Another bias tee (that’s three now), but at least this one does not need to dissipate large amounts of current – it’s I^2R that causes the heat – and current is squared – damn!!!

Having three tees with the DC voltage drop causes quite a lot of design issues, and it took some time to identify the right components to do the job. There are chip inductors, air coil surface mount inductors. Coilcraft even make some core based smt inductors. The challenge is to get something with a high enough inductive reactance that does not take up too much space, and has not effectively become a capacitor because it is above SRF. Some devices are designed to be used above SRF and that’s ok, but most inductors are not, so this does provide a limit. The air coil ones just don’t have the reactance, even though they have low DC resistance.

The amp provides 26dB gain on 50m and 25dB on 2 and 70. It has a noise figure of 0.5dB. That is really nice. This should be a good quality preamp.

The 5V supply requirement of the amp meant that I selected 5V relays. I did not want to add some BJT device with a zener regulator to bring down 12V (or 13.8V or so) to 5V. Extra complexity for the preamp, duplicated on each one. It will be far easier to get a 5V DC-DC step down converter off ebay, plug some Anderson Powerpoles on it and run that from a 12V-15V or even wider supply. So this is the plan.

The DC current requirements of the amp and the two non-latching relays are 160mA. I needed to ensure that across all the bias tees to the amp (thats three remember!) that I was getting at least 4.75V there. I did contemplate using the amp in 3V mode, but that needs a zener plus a BJT, so I tried to avoid that. In the end, I can get 4.87V to the amp, and that is well within spec.

Here’s a look at a Schematic for the preamp.

Schematic for preamplifier for 6, 2 and 70

It should be noted that component values will change for each band. It does not seem possible to retain simplicity and have a preamp for all three bands, but at least I should have a common PCB board design.

I will need a separate schematic for the radio side bias tee. I will talk about this more on a future post, but it is going to need to take the TX indication out of the radio, use a PIC chip to TX inhibit for 30ms or so, and switch the DC – through a BJT and then onto the bias tee onto the coax.through two)

Part 2 can be found here.