Scorpion SA-680 Screwdriver Antenna Setup for 80m – 15m
This article will cover the completion of our Mobile HF project. Our installation is fairly complex, so we’ve broken the project into several phases:
The first step in this part of our project was to add upgrades to our Scorpion SA-680 Screwdriver Antenna to enable it to be used on 160m.
Screwdriver Antenna Configured For 160m
Ron at Scorpion Antennas makes an add-on coil unit to enable his 80m – 10m antennas to work on 160m. The picture above shows the add-on coil installed along with a 3 ft rod and Cap Hat. The coil adds additional base loading inductance to enable the antenna to work on 160m.
Antenna Base With Shunt Coil Switching Unit
A different shunt coil is required to properly tune up the 160m configuration. Ron makes a very nice shunt coil switching unit (the grey box attached to the base of the antenna in the picture above) to allow multiple shunt coils to be used.
Antenna Shut Coil Switching Unit
The unit contains two shunt coils (one for 160m and one for 80m/40m) and a 12V relay. The relay switches in the appropriate shunt coil are under the control of a switch that I’ve added to our truck’s console. After properly adjusting both shunt coils, I was able to get a good match (SWR < 1.4:1) across the 160m, 80m, and 40m bands.
The next step in this phase of our project was the installation of a mobile HF amplifier and some accessories to make the operation of our mobile HF station easier. Alan Applegate (K0BG), author of the excellent K0BG.com website on mobile HF, points out that installing an amplifier in a mobile application is not a trivial project. One must pay a great deal of attention to the following areas:
- Beefing up the vehicle’s electrical system to be able to supply adequate 13.8V power (a 500W mobile amplifier will require 60A – 80A of sustained current when transmitting at full output and may draw close to 100A on peaks).
- Proper physical mounting and adequate cooling to dissipate the heat generated (a 500W amplifier will generate approximately 500W of heat when operating at full output)
- Proper choking of antenna control leads to ensue that conducted RF does not get into your vehicle or electronics (this area was covered during the installation of our Screwdriver Antenna)
- Drive/output power and SWR monitoring to ensure that the amplifier is not over-driven and is working into a properly tuned and matched load
I am going to cover each of these points as they were handled in our installation.
Secondary Battery And Fuses
It’s important that your vehicle’s alternator is capable of supplying enough current to operate the amplifier and the rest of your vehicle’s electrical system with overloading or major drops in voltage. Fortunately, our F-150 Truck came with a 150A alternator from the factory. The next problem to solve is to provide 100A+ of peak current during transmit without excessive voltage drop (you want to have no more than 0.5V of drop between your primary battery and the power terminals of your amplifier). The easiest way to achieve this in our application was to install a secondary battery in the bed of our truck. We choose an Optima Red-Top series battery for the secondary battery and as a replacement for our truck’s primary battery. The Red-Top series provides very high current for short periods of time (ex., engine cranking). This profile is ideal of supplying a mobile amplifier. Both batteries are connected in parallel with custom-made 2 ga cables for both +13.8V and ground. The batteries must be identical when connected this way to ensure that differences in operating voltage do not result in uneven charging. For safety reasons, It is also critical to properly fuse the connections between the batteries at both ends and in both the +13.8V and ground leads! We used high-current fuses (the insulated holders to the right of the battery in the picture above on both ends of the battery cabling. If either cable becomes shorted to the other or to ground, the fuses by the batteries will blow and prevent a fire. It’s also important to securely mount the batteries and associated cabling and properly protect the cables. We used insulated cable clamps and convoluted tubing to accomplish this.
Ameritron ALS-500M Amplifier And Radio Interface
The next step was to select a location for the amplifier that enabled good airflow around the unit and securely mount it to the truck’s floor. The spot we chose was under the flip-up rear seat of our truck. We used The mounting location at the corner of the seat, ensuring good airflow around the unit. We also made an aluminum plate that sits between the amplifier and the carpet in the truck to ensure that cool air can circulate under the amplifier without being blocked by the carpet. The Ameritron ALS-500M Amplifier we used comes with mounting brackets that allow it to be securely screwed to the floor of the truck so that it does not become a safety hazard during a quick stop, etc. We also installed an Ameritron ARI-500 Radio Interface Unit, which provides automatic amplifier band switching and a keying interface for our Icom IC-7000 Transceiver.
Amplifier And Accessories Under The Rear Seat
The picture above shows the location of the amplifier under the rear seat. Note the clear path that the vents in the case have to the air which circulates within the vehicle. There are air conditioner vents behind the front seats in the center console, which can direct cool air on the amplifier during warm conditions.
Power Distribution, Antenna Controller And Wattmeter Sensors
The picture above shows the layout of our installation’s power and accessory electronics. Note the two high-current fuses protecting the power connections between the amplifier and the secondary battery. We also installed a RIGRunner 4005 Power Distribution Block to supply 13.8V fused power to all our accessories. Again, we used black convoluted tubing to protect all of the cabling and dress up the installation.
Wattmeter Sensors And Screwdriver Antenna Controller
The picture above shows the remote sensors (left) for the Elecraft W2 Wattmeter that we are using in our installation. A separate sensor is used on the input (200W range) and the output (2Kw range) side of the amplifier so that we can accurately set our drive power as well as monitor the amplifier’s output power and the SWR, which is being presented by our antenna.
The TuneMatic unit is an Automatic Screwdriver Antenna Controller. This unit senses the frequency of our radio and automatically adjusts our screwdriver antenna to provide a good match. It also has auto-tune capabilities and includes an amplifier key line interrupter relay to ensure that we do not transmit high power into the antenna while it is being tuned.
Control Layout On F-150 Console
All of the controls for the Amplifier, Screwdriver Antenna Controller, and Elecraft Wattmeter are mounted next to the driver on the F-150’s shifter console. We used heavy-duty Velcro strips to mount everything. The device in the right foreground is a Remote Control Unit for the Amplifier. The device in the left foreground is the control head for the TuneMatic Screwdriver Antenna Controller. Just behind the Screwdriver controller unit is a lighted 13.8V switch which we installed in the console to switch the shunt coil relay between the 160m and 80m/40m shunt coils. Finally, the unit in the background is the Elecraft W2 Wattmeter.
Antenna Controller, 160m Shunt Coil Switch, And Amplifier Remote Unit
The TuneMatic Antenna Controller will automatically adjust our screwdriver antenna with a simple touch to the Tune (TU) button when the radio frequency is changed. This unit can also be used to manually move the antenna up or down to fine-tune the match. There is also an auto-tune function which works well. An antenna controller such as the TuneMatic makes changing bands and frequencies much safer and easier to do and ensures one keeps an eye on the road.
The Ameritron ALS-500RC unit provides a switch to enable or reset the amplifier if it should trip and a remote current meter, which shows how much current the amplifier is drawing.
Elecraft W2 Wattmeter
The Elecraft W2 Wattmeter worked out well in our mobile HF application. One can easily select the input or output sensors, and its auto-ranging features provide accurate power and SWR readouts. It also has an LED brightness adjustment which is nice when operating at night.
Voltage Monitor
It’s important to be able to monitor your vehicle’s voltage when using an amplifier. The heavy current demands of an amplifier at full power output can cause significant voltage drops, especially if the vehicle is idling and other power accessories like de-icers or seat heaters are in use. I found a simple and inexpensive solution for voltage monitoring – a unit that plugs into the cigarette lighter jack in the vehicle. The unit has an easy-to-read display and does not draw much current, so it can be left plugged in when our truck is parked.
It took a little time to set up the TuneMatic Antenna Controller to quickly adjust the Scorpion Screwdriver antenna on all the bands from 160m – 10m. The instructions which come with the unit explain this process, and it is not difficult to do. The TuneMatic must be configured to work with your particular screwdriver antenna when it is first installed. This involves setting some option switches and adjusting a pot inside the TuneMatic unit. Again, the instructions cover the setup steps well.
Icom HM-151 Microphone
The Icom IC-7000 Transceiver is an excellent radio for mobile HF applications. Unfortunately, the quality of the transmitted audio with the “stock” Icom HM-151 microphone, which comes with the radio, is less than ideal. To solve this problem, I sent my HM-151 microphone to Bob Nagy (AB5N) for some upgrades. Bob performed several upgrades, including replacing the element with a higher quality unit, installing a heavy-duty PTT switch, weighting and vibration deadening the housing, and other mods. After adjusting the equalization in the IC-7000 to match the new element, we are getting some very nice reports on our audio quality from the stations that we are working while mobile.
You can click on the above video to hear what our mobile HF station sounded like in Europe during initial testing with the new amplifier and upgraded microphone. We were still adjusting the audio settings when the recording was made, but it will give you an idea of what the setup sounds like on the air.
There was a lot of integration and working with the folks at all the companies who supplied the components for our mobile HF project. Ron Douglas at Scorpion Antennas, Mike at Ameritron, Jim at TuneMatic, Bob Nagy (AB5N Microphone Upgrades), and the folks at DX Engineering were all very helpful in answering our questions and getting everything to work together.
Scorpion Whip Quick Disconnect
We are operating across a wide range of bands (160m – 10m), and I’ve found that it is best to use a range of “whips” on our screwdriver antenna to cover all the bands. The Scorpion Quick Disconnects make changing “whips” a snap. Our “go-to” configurations are as follows:
- 160m – We use the add-on 160m coil plus a 3 ft rod with a Cap Hat. The Cap Hat makes the rod appear electrically longer, improving overall efficiency on the Top Band. This combination allows the antenna to be tuned for all but the top 50 kHz (above 1.950 MHz) of the 160m band.
- 80m – 15m, including 30m and 17m – We use a 4 ft rod with a Cap Hat. This combination is very efficient, and our results on 80m have been particularly good. I am able to work DX from New England, USA, into Europe on 80m with 100W (amplifier off) using this combination. The 4 ft rod/Cap Hat combo is electrically too long to tune above the 15m band. The 3 ft rod/Cap Hat will tune up on the 12m band but not on the 10m band.
- 12m and 10m – We use a 6 ft whip for these bands.
We really like the combinations which utilize a Cap Hat. These setups are definitely more efficient than the 6 ft whip, and the overall height of the combination is low enough to stay out of the low tree branches here in New England, USA. The Cap Hat combinations allow less of the screwdriver antenna’s coil to be used. The coil is one of the largest sources of loss in a properly installed screwdriver antenna setup, which is why the Cap Hat/short rod combinations work so well.
It’s nice to have the extra power when operating from our truck, and I find that I can call CQ and sometimes generate a pileup while operating mobile! I’ve also been working quite a bit of DX from our mobile HF station (95 DXCCs worked so far), and the improved antenna and the added power have also helped in this area.
We hope you have enjoyed our series of articles on our Mobile HF project. We have learned a great deal doing the project, and we’ve made over 600 contacts from our truck along the way, with many more to come.
– Fred (AB1OC)
