I'm sure there are others out there that travel with mobile HF also. The question is - when you get to where you are going what do you use for an antenna?

If you are like me and have screwdriver antennas you've run cables out to your truck for coax and tune control of some kind.

Ever since I got on HF in '02 I've been running screwdriver antennas and have set up like this - portable in a number of locations from beach houses to Field Day. It has always required two cables.

Recently I have been just running coax and foregoing the second cable for the tuning. I used an Elecraft KX3 radio as my antenna analyzer. I would set it up in the truck to tune one of my screwdriver antennas. Then I would switch the coax around to the long run back inside and run my radio from inside. That was a PITA to tune around - like on 75m where the bandwidth is so narrow with a screwdriver. If I wasn't careful where I put the SWR dip it would mean a few trips back and forth depending on where I operated on the band.

I recalled an article from Phil AD5X from a while back where he was inserting DC power over coax. If you think about the way a screwdriver tunes - it is 12v power on and off in both polarities. One polarity raises the antenna (lower frequency) and the other polarity lowers the antenna (higher frequency). Of course, there are "tune controllers" out there that work off a reed switch that counts pulses, but the movement of the antenna is simply + or - 12v power.

Back around January this year I was enjoying chicken wings and a few beers with a buddy of mine all the while drawing up schematics and talking DC power and RF theory. The subject was this exact device - how you reverse the polarity of DC on a circuit that always passes RF and allows DC only on a portion of it?

Our napkin schematics certainly made this one much more complicated than it needed to be.

Think about it for a second - the antenna does not care if there is DC there or not, only the radio. The DC is also not present (in this application, the article linked above is a different story) all the time - it is only there during the tuning process. Therefore, the extraction circuit is entirely irrelevant so long as the basic DC blocking and RF choke is there.

The trick is the dual polarity and the DC filtering on the injection circuit. The first problem is that the shield of the coax has solid continuity throughout the circuit from the radio chassis to the antenna ground. In reverse polarity this means positive DC needs to flow on the shield of the coax. You can't use the same 12v DC to power both the radio and the tune control because doing so would short out the power supply in reverse polarity. If they are independent circuits there is no way to short the circuit between two power supplies - they are floating - problem solved. The other problem is the DC filtering is polarity-specific. Easy enough - put the filter on the input side, before the injection DPDT switch that controls the polarity.

Below is what this ended up turning in to.




The method that I decided to use was to isolate the current of the tune motors to local power - vehicle 12v or separate battery - and use relays in the extraction circuit to do the switching. In this manner only the current for the relay coils is present over the coax. Then I used power diodes to direct the relays so only one is powered in each instance of power fed through the coax in each polarity.

The original DC blocking capacitors I used were 650v rated. If you read the schematic in the article linked above - there were 3x 2kv caps used in parallel for DC blocking. The caps I used started out OK, however I went to key up on 75m at 100 watts (after tuning on 5 watts) and I blew the capacitors. My SWR immediately went haywire. I did not get a screen shot of the insertion loss prior to blowing the caps, but here it is afterwards:



If you notice - around 75m (3.5MHz) the insertion loss is about 6dB. Every 3dB is a double in power, so right there I was not going to get an acceptable SWR no matter what - 4:1 at best.

I found some 3KV rated caps on ebay and replaced the 650v rated ones with these. Problem solved.

Here is the insertion loss now:




It is hard to get any resolution on the 15dB scale.







I knocked the scale down to 1dB for better resolution. I think the service monitor is struggling to display that correctly - it shouldn't be a square pattern. Regardless, though, the loss is about as small as the fart from a gnat. To add to it - this is the whole HF spectrum + 6 meters. Sweet!

This set up allows me to run 1 coax cable for both the RF and screwdriver tuning. I use RG-174 (LMR-100) for a pass through under door jams and through windows, then RG-58 for the main runs. All connectors are BNC as they are quick, low loss, and for running 100w are more than adequate for power capacity.

I will see if I can get a schematic drawn up of what the final circuitry is.

 

I don't do HF in the vehicle. Now for Field Day or Summits on the air I would like a good portable rig. If the right 706 and tuner were to come around or an 857 and tuner I'd jump on it. If I get set up I'm pulling wires and running a dipole. I need to get some ferrite cores to build some baluns.

Where did you find your service monitor?

 

The service monitor came from Ebay about 6 years ago. I was installing commercial systems in the 450 band and needed to be able to do loss measurements, antenna measurements, and tune band pass filters. This one has the whole 9 yards - tracking generator, spectrum analyzer, and it has a built in function generator with all the different waveforms. This is the Agilent 8935 E6380A model - the high power RF analyzer. I can run 100 watts in to it no problem. For checking spectral purity it is fantastic.

The 857D is a nice radio. I own one. I ran that in 3 trucks and was installed in to the F350 when I drove the truck off the dealer's lot. The bad news - the receiver crystal filters are shot. It has been in a box for years. I would research it a bit. The issue is that the circuitry that the 857D (and 897D - they are the same circuitry, just different boxes and interfaces) uses puts DC voltage on the filters. They should not have DC voltage present. Over time the filter poles oxidize and that is that. The fix to it is to do some board modifications to install surface mount capacitors around the filters to block the DC. I, for one, hate SMT's. I have done a lot of board repairs and mods with SMT's, but they drive me up a wall. One of these days I will get around to the filter replacement and board mod on the 857D.

I just pulled a dead Kenwood TM-V71A out of a box and fixed it - SMT fuse blew on the mic circuit ground. I've been using it for 2 weeks now - as good as new once again.

 

Yesterday I bought a vintage Kenwood TS-520. A week ago our club had a tailgate party. This radio was there. It has some new tubes and spares. I need to figure out a good antenna for home. I also have a Yaesu FT-450D that is a nice little rig for HF. I have been making a few QSOs with it.

Friday I also tried on a new climbing harness. I'm 40 years old and the young guy in the club. I am needing to climb a 140 foot tower to replace a feed line. Also will inspect the tower and see how the antenna is working.

 

If you were closer I'd offer to help with your tower work. I've been climbing towers for years - everything from the fiberglass vertical UHF antennas I used to install on the commercial systems to multi-band HF beams.

I acquired a very early Mosley CL-36 from one such job - whoever took it down got to take it home - it was the hardest tower job I have ever done - 90lbs of antenna, over 30ft boom, and 40ft elements that got tangled up in guy lines on the way down. It was partially dismantled before it hit the ground - by necessity to get it out of the guy lines.

One of the coolest tower jobs I ever got to do was over 200ft. The top of the tower had equipment that we needed to get up and over so I had custom tower brackets made for a small 20ft tower to go up on top for my antenna to go on. We went up, with the small tower and brackets in tow, in a man basket off of a 300ft tall lattice-boom crane. I always used 7/8" Heliax (another perk of that work - I got to keep the spool ends ). The coax was bolted down to the structure with heavy duty clamps. That was the easiest coax-hanging job ever - the crane and basket did all the climbing - and I had all my hardware resting in the basket and not off my climbing harness.

The 450D is a nice radio. My main radio for years was a Kenwood TS-2000 (still have it, blew out the 2m finals transmitting in to a poorly matched antenna though - another repair project for the work bench at some point). Several years back a friend of mine got the 450D. We did a receiver comparison between that and the TS-2000. The 450 was noticeably more sensitive and quiet. I could hear signals on the 450 that weren't intelligible on the TS-2000. I guess that is what you get for a quad conversion, up conversion, superheterodyne receiver in the TS-2000. It is hard to complain, though, because of the rest of its capabilities - frequency/band range. It has been a good radio - I still use it and am not getting rid of it.

My main radio now, and mobile radio, is a Yaesu FT-991A. I found out around Christmas time this year that it does Fusion also. I got it for the HF through 440 capability and that it has a color spectrum display. I have started using Fusion a bit. It's fun. I do prefer DMR to Fusion, though.