Other VHF  & Above Technical Errata (rev 04-10-05)

1.  Transverter interfacing.  (Careful with that Axe, Eugene) Lot's of guys run into trouble when they try to get ready for the higher bands.  Many modern-day rigs do not have built-in transverter ports, so the interfacing job can be challenging.  I certainly don't know enough about most popular transceivers to be of great help, but here's what I did with my IC-706MKII.  I use it as an IF on 144 MHz to interface with transverters for 903-10368 MHz.  My transverters all want 2-3W of RF drive on transmit. (and I haven't blown any up yet!!). 

The major task of interfacing a transverter with the 706 is that of making sure you have a method of reducing the RF output on 2 meters to a level compatible with your transverter.  Most popular transverters can live with 2-3 watts of drive, and the nice units from Down East Microwave (DEM) can be ordered for nearly any level you would want to use.  For the 3W level, I built a simple negative power supply board using the 7660 voltage inverter IC.  The 706 has an external ALC input, which will limit the transmit power level when it received a negative voltage between -5VDC and 0 VDC.  I just used some perforated PC board material from Radio Slack, and took a 7805 regulator to drop the vehicle voltage down to +5VDC.  Then this voltage is fed to the 7660, which turns the +5V into -5VDC, at low current.  The ALC input doesn't load down this supply very much, so even though the 7660 only puts out 20mA, max, things will be OK.  For a data sheet on the 7660, check http://dbserv.maxim-ic.com/quick_view2.cfm?qv_pk=1017.  They even have schematics on there, if you browse around.  Take the output from the 7660 to a 10-turn pot, and run the wiper into the ALC input.  Of course, you should also add some .01uF bypass capacitors, and some RF chokes, or ferrite beads to keep thing cool from RFI.  I set my IC706 for a low level using the built-in power reduction feature, and then keyed up the rig on CW, and set the pot for 2.5Watts output on 144.100 MHz.  If you are thinking about this carefully, you may ask...why not just use the IC706 built-in power control?  Well, the answer is that it produces spikes in the RF output amplitude, which can blow up your favorite transverter.  I think the reason is that internally, the 706 filters this line with a capacitor, which slows down the response time.  It only takes a few uSec to zap your expensive transverter, however. 

I recently experimented with my FT-100 to see what was required to interface it to my DEM 222 transverter, using a 28 MHz IF.  I found that even with the RF power on HF turned all the way down, there was an 80W RF power spike which occurred whenever I pressed the mic PTT switch.  It only lasted 5 milliSeconds or thereabouts, but it's enough to toast a receive mixer stage (I didn't do this, fortunately).  Many guys have blown transverters by failing to realize the existence of this RF power spike.  After the initial spike, the RF level was a well-behaved 3W, which was about what my transverter requires. I have tried the addition of negative ALC voltage into the FT-100 ALC input, but this did not work.  Well...it appeared to work at first, but after cycling the PTT a 2nd time, there was no RF output.  Apparently, there is some sort of hysteresis with this ALC.   In other words, you can adjust for a low power output at first, but the needed ALC voltage is different after unkeying the FT-100, and re-keying it.  It's possible that the FT-100 requires more current than my -5V supply has avaialble (10mA).  I had to abandon the ALC approach, and find another solution.  Fortunately, using the DEM 222 xvtr with a specified 3W IF input, a fairly robust attenuator is used at the transmit IF input.  As long as one can be sure that the transverter is keyed into transmit BEFORE the 28 MHz power spike occurs, everything works out OK.  Doing this required some fancy sequencer work, however.  My rover partner, ON4IY designed a slick delay generator PCB, which does the trick.  It is small, and accepts the FT-100 microphone cable as it's input...and mates with the FT-100 transceiver, using a cable like the microphone RJ-8 connector.  This board allows us to just squeeze the mic...it then has auxiliary output to key the xvtr and the power amplifier.  After about 50 milliseconds, it then keys the FT-100 radio, and all is well.  We were quite worried about testing it, knowing that we had not eliminated the 80W RF power spike, but it was getting late on the night before the contest, and we just had to try it...expecting that the DEM input attenuator could handle the large peak voltage.  It worked admirably...but only because the ON4IY board prevented the FT-100 from transmitting until after the xvtr and PA were comfortably in the transmit mode.  I'm surprised that someone hasn't put boards like this in the market.  It was rather tricky, getting the mic ground circuit totally isolated from the outside world, though.  It seems that if anything is connected to the mic ground, all sorts of hate, discontent, and AC hum get on the transmit signal.  This is bad, and we had to provide an isolated set of relay contacts for this function.  (this can be tricky at 11pm, the evening before the contest...hi).

Another successful approach that can prevent RF power spikes from damaging your transverters is to disconnect the DC power from your IF exciter power amplifier.  This is a proven approach for the IC706 transceiver, and it's probably applicable to the FT-100 and other radios as well.   This idea is from K6LEW (now K3CB)who spends lots of effort helping some of the K8GP rovers configure things for the contests.  Owen says that it's necessary to go inside of the IC706 to do this, but that it isn't difficult to find the right place to disconnect DC power.  About 100mW is still available from the RF output connector, as the RF leaks past the disabled PA stage.  The beauty of this approach is that it's easy to restore normal operation, when driving transverters is no longer desired (preserving resale value of the IC706).  The drawback is that is disables the transceiver from being used for full power operation on the IF frequency (unless switching is added). 

Another area which is dangerously ignored by many VHF and above operators is that of sequencing.  Lot's of guys have gotten away without doing, it, but most have already, or will soon be paying a price for this shortcut.  All vhf systems that I am aware of should use sequencers!  Think about it...if you try to key up your transverter, T/R switch, power amplifier, etc from the exciter, the RF will already be coming out of the radio before the external gear has had time to switch.  Yeah, I know about the QST product reviews that show a variable delay in the CW RF output...but I don't think any of these rigs provide a delay in the RF output for SSB.  (The IC551D that I had many moons ago actually did have a bucket-brigade delay line inside which did perform this job...but it was mind boggling monitor yourself in headphones with this delay happening).   RF Relays can take up to 100mSec or more to fully close, and hot-switching most RF relays will damage them (now you know where many of those "bargain" RF switches at hamfests come from!).  Anyways, what I am trying to say is that to be safe, you really need an external foot-switch or toggle switch of some sort to trigger your VHF-microwave station into the transmit mode....you can't reliably use the microphone PTT contacts, unless you run them through another box of some sort.  If you have any information about radios which implement an RF delay on both CW and SSB, please email me at wseab at ieee.org.  Tnx.

I know this sounds inconvenient, but I think it's the correct way to do stuff.  If you really want to be crafty, you can build a microphone interface unit, which will accept the mic PTT contacts, and then drive the sequencer, which then drives your exciter to place it into the transmit mode.  I like have a separate toggle switch for CW, but many operators stick with the microphone PTT switch to trigger the station.  Most high-performance stations still use a foot-switch, to my knowledge.

One interesting note on the 706...if u use an external foot switch or toggle switch to place the rig in the xmit more, the microphone still works without u having to squeeze the mic PTT switch.  This is convenient, but it is not foolproof.  U could still cause problems for some xvtrs by forgetting the proper sequence and squeezing the microphone too early, or without throwing the toggle switch first.  It's just too easy to make mistakes like this in the heat of a contest...so take the time and make things foolproof...or as W1GHZ likes to say...fool-resistant (!)

Anyways...you need to take your PTT source...and trigger a sequencer from it.  The sequencer should then 1) first close the antenna T/R switch (and the IF relays, if your transverter uses them at the RF in/IF out),   2nd, the sequencer should  key any power amplifiers being used, and 3) lastly key up the exciter...in my case the IC706.  Most radios that I know of have a keying input line for remote transmit activation.  How long should the time delay be for these steps?  It depends on your specific equipment, but typically, 50 milliseconds is long enough between each of the above steps.  DEM makes excellent sequencers, although they are a bit large.  DB6NT has a nice series with various current output levels up to >10A DC current.  These latter units have less flexible voltage levels, and they use P-channel MOSFETs to switch large currents.  The real beauty of the DB6NT sequencers is that they are a little larger than a postage stamp!  Very nice to add inside a power amp or something.

2.  DC Switching.

Speaking of P-Channel MOSFETs, these puppies are awesome!  Using them, you can implement a small high-side DC switch, using a ground-to-transmit-high-Z input.  All you need is about a 22K pullup resistor between the +V input (the source), and the gate lead (control input).  When you ground the gate, high-current DC is available at the drain lead.  (use a fuse, cause if you don't, the low Rds-ON of a good P-channel FET  can produce a nice pyrotechnic display, and smoke your equipment!!!)  Anything that provides a ground will trigger the largest P-channel FET.  Don't forget to use steering diodes, though, if you plan to switch multiple things from one set of PTT contacts.  Also, many modern rigs have a very limited keying output signal which can only sink a small amount of current...perfect for P-channel MOSFETs.  The best FET I have found is the IRF4905 device from International Rectifier.  These bad-boys will switch 70Amperes!! ...and they are still very high-Z input...what's not to like?  They are available from Digi-Key (albeit at highly inflated prices).  For a good deal on these FETs, go to an electronic supplier like Newark, Future, or Avnet where you can get them with a credit card for about $1.00 each!  These things are the control element-of-choice for solving many switching problems.  The Rds-on is lower than most pieces of wire!!  It's like a true solid-state relay for positive supply currents.  Cool

Have fun, and get those transceivers talking on the microwaves.  We're waiting to work you!

3.  Dual-Band Feeds.  Dual-band feeds are really handy for roving.  W5LUA, W5ZN, and others have done a great job describing these feeds in one of the Microwave Update proceedings.  It's wonderful to get 2 dishes for the hassle of just one.  If you want to put some horsepower into that feed, however BEWARE.  The DEM 5.7/10GHz feed has pretty good isolation between the 5.7 GHz port ant the 10 GHz port, when you are transmitting on 5.7GHz, but on 10 GHz, there is a lot of leakage into the 5.7 GHz rig.  If you are running more than 100mW, I strongly suggest that you consider a simple modification to your hardware.  I am using +12V on both band's T/R relays, and I ground one side to close the relay for transmit.  (this is a common way that folks use).  Here's what you can do...if you also are doing a ground-to-key on your RF switches.  Run a wire from the ground-to-xmit terminal of the 5.7 GHz relay...through a diode with the cathode away from the relay.  Run this line to the control line feeding the 10 GHz relay.  Then, do the same thing from the control terminal of the 10 GHz relay, through the diode, and over to the control circuitry of the 5.7 GHz hardware.  Now, Whenever you transmit on either band, you will throw both RF relays into transmit, but you will only key-up the band that you are transmitting on.  I find that I can leave the GaAsFET preamps fired up without any problems, but some guys like to switch them off during transmit.  I don't think it matters, as long as you keep the preamp inputs down to 0dBm or less.  (some have reported +20dBm as the threshold of pain...but you could damage the NF with less).  This is a really big problem for me on 2304/3456.  I have a dual-band-feed for these bands, and the isolation from 3.4GHz to the 2.3 GHz port is very low...like 12 dB or so.  If I light off my 40W on 3456 into this puppy, I get more than 2W of juice into the 2304 preamp.  Ouch!!%&*#(@__+!!  Fortunately, I haven't done this, but I plan to install it before fielding my 2.3/3.4GHz dish assy again.  GL, and protect your precious preamps!!  Allegedly, the newer dual-band feeds from Directive Systems (& Down East Microwave) are better tuned to optimize isolation, but you could still get reflected RF power from nearby antennas or ground-clutter that could cause problems.  The presently avaialble dual-band feeds from Directive Systems are advertised with lots of isolation between 10 GHz and 5.7 GHz...so this may not be a problem for those of u using the newer feeds. I suspect it is a rather critical adjustment, however, so if u are building ur own feed and/or experimenting...beware the possible leakage.  Too much RF in the wrong place can ruin ur whole day. :(

4.  More Info on dish feeds.  Click Here


73, de W3IY/R