We have made some 250+ contacts using the Mobile HF setup in our Ford F-150 Truck. Our initial setup used an Icom IC-7000 Transceiver and HAM Stick Antennas. This combination enabled us to work quite a bit of DX from our truck (56 DXCC’s worked mobile HF to date). It is surprising how well a properly installed Mobile HF setup works. For more information on our initial Mobile HF installation, see our other articles here -
It has been part of our plan to enhance our Mobile HF installation to include a Screwdriver Antenna and a Mobile Amplifier. This article cover the first of these upgrades.
We chose a Scorpion SA-680 Screwdriver Antenna for our project. Scorpion mobile HF antennas are among the best on the market in terms of quality and efficiency. The SA-680 covers all HAM bands from 80m – 10m (160m operation will also be added to this antenna – more on this in a later post). The SA-680 is also a good choice given our plans to add a mobile HF amplifier to our truck (the SA-680 is rated for 1500 Watts SSB PEP). We ordered our antenna with a flat black powder coated finish to match the color scheme on our truck. Scorpion antennas can accommodate a variety of whips and we have both 6 foot (supplied with the antenna) and 8.5 foot whips available. We also ordered a 3 foot long rod with a Cap Hat and this is the combination that we are currently using. This arrangement features improved efficiency due to the top loading of the antenna provided by the Cap Hat and a reduced height profile which is perfect for avoiding the low tree branches here in New England, USA. Ron Douglas, NI7J owner of Scorpion Antennas has been a great help providing lots of good advice to help us to install his SA-680 antenna properly.
The Scorpion SA-680 Antenna is a beefy unit and weights about 18 pounds. This antenna requires a strong mounting system for safety and reliable operation. We chose to mount our antenna in the bed of our F-150 Truck using a fold-over mount from Breedlove Antenna Mounts. This mount is made specifically for the Scorpion Antenna and uses 1/2″ thick aluminum plate with reinforcing bars that mount under the truck bed to ensure that the mount is rigid and does not crack the truck bed due to the load of the antenna. As you can see from the picture above, we cut the plastic bed liner which protects our truck’s body to allow the base of the antenna to sit on the truck body’s sheet metal. We used star washers between the mount and the bed to ensure that the mount made a good RF connection with the bed of the truck.
One of the nice features of this mount is its ability to be folded over to 90 or 45 degrees. This is done by loosening two large allen screws on the mount. The picture above shows the antenna folded over with the Cap Hat/Rod removed. In this position, we can close the roll cover on our truck’s bed to completely cover the antenna. This is great for taking the truck through the car wash or when we want to protect the antenna from the winter weather here in New England.
Removal of the Cap Hat/Rod or an attached whip is made easy by Scorpion’s Quick Disconnect. The ones on our antenna are made from stainless steel and allow easy removal of the Cap Hat Rod or a whip.
All Screwdriver antennas require control cables to operate a motor which raises and lowers the antenna to change the length of a base loading coil. This is how the antenna is tuned to operate on different bands and frequencies. The motor and associated control leads operate at a high RF potential relative to the vehicle’s ground. This is also a problem for the outside shield on the coax cable feeding the antenna. The RF potential on the control cables and feedlines must be choked or it will enter the vehicle and cause all manner of RFI problems. Proper RF choking is especially important in our installation as we plan on running high power via an amplifier. To ensure proper choking, we built a series of RF chokes for the six control lines from the antenna – one pair for the 2 motor leads, one pair for the 2 pulse count leads that are used to sense the position of the antenna, and one pair for a future shunt coil relay to enable 160m operation. These chokes were built according to the information on K0BG’s excellent Mobile HF website. Each pair of control leads were run through two separate chokes at the base of the antenna (two chokes were used to due to the planned high-power operation with an amplifier). Two chokes were also used on the RG-8X feedline. For the initial installation, the supplied shunt coil (copper coil on the left side of the antenna base) was used to match the antenna on 80m and 40m. The coil was adjusted using the procedure on K0BG’s website to achieve a good match on these bands. Also note the ground strap (visible to the left of the fold-over base). This connects to one of the button head screws at the base of the antenna and to the ground point on the antenna mount to ensure a good ground between the antenna and the truck.
Another set of control cable and feedline chokes were made and installed at the point where the antenna cables and feedline enters the vehicle. The picture above shows some additional detail on how the chokes are made. It is important that the chokes be at the same point on each of the control cables and coax feedline so that any RF induced on the cables does not couple from one cable to another and bypass the chokes.
I decided to do some additional work on the bonding of the bed of our truck to the rest of the F-150. This involved installing 4 bonding straps between each corner of the F-150’s bed and the frame. I used the excellent strapping material from Electric Motion for this purpose. This strapping features eyelets which are installed every 3 inches along the strapping material. This made attachment of the strapping to the F-150’s bed and frame easy to do via self-taping stainless steel screws and star washers. A liberal coating of Ideal NOALOX was used on each of the screws and washers to protect against corrosion. Also note the convoluted tubing which houses the antenna control leads and feedline running along the frame. The tubing protects the antenna’s cabling and feedline from the weather and enables secure mounting to the vehicle’s frame.
The final step in the installation was to connect the antenna control cables to Scorpion’s antenna control switch and mount the switch on the console with velcro strips. This switch is used to raise or lower the antenna, changing the length of the loading coil to tune the antenna for different bands and frequencies. I also installed crimp-on connectors on the RG-8X feedline and connected it to the radio and to the antenna. A quick check of the antenna’s SWR on 20m confirmed that the antenna and feedline were working correctly.
I was able to make contacts on the 17m, 20m, 40m and 80m bands with the new antenna last evening and it works great. I am particularly pleased with the antenna’s performance on 80m. I made several contacts on this band out to about 2,000 mi and was receiving signal reports ranging from 58 to 59+20dB. These results are very good considering the short length of the antenna’s Cap Hat/Rod (only 3 feet) and that I was using only 100W.
*** Safe operation of your vehicle requires your full attention on the road. You SHOULD NOT try to tune your antennas or your radio while your vehicle is in motion. Safety requires that you perform these actions only when your vehicle is stopped and parked safely. ***
The antenna is easy to tune manually. One simply sets the radio to the desired band/frequency and then adjusts the antenna up/down until the maximum reading on the radio’s S-meter is obtained. This usually gets you to within a “coil turn” of the optimal tune-up. I then key the radio up and adjust the antenna up or down a bit to optimize the tune to the lowest SWR as indicated on the radio. The antenna’s tuning is not critical on 20m and higher bands. It is fairly sharp (due to the short length) on 40m and 80m so final adjustment to minimize the SWR is important on these bands.
It’s great to have the full set of Amateur HF bands available in the truck with the new antenna. Performance seems comparable to the Ham stick antennas I was using previously on the 20m and higher bands. I would say that this indicates that the efficiency of the Scorpion SA-680 is significantly higher than the Ham stick antennas because the Ham sticks where mounted dead center on the roof of my truck and about 3-4 ft higher than the Scorpion. While this location performed reasonable well on 20m and higher bands, it was not a very practical mounting location due to height problems and the difficulty in getting at the base on the antenna to change or remove the Ham sticks. Performance is also much better on the 40m and 80m bands with the new antenna. I would say that the new antenna is also somewhat quieter than the Ham sticks were. This is probably due to a combination of being further away from the engine and cab noise sources plus benefits due to the additional bonding work.
The next stage of our Mobile HF project will be the installation of a mobile amplifier, automatic antenna controller and 160m band add ons to the Scorpion antenna. I hope to work some of our readers on the HF bands from our truck soon!
- Fred (AB1OC)