Mobile HF Installation – Part 4/4 (500W Amplifier, 160M and Accesories)


Scorpion SA-680 Screwdriver Antenna

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

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

Antenna Base With Shunt Coil Switching Unit

A different shunt coil is required to get the 160m configuration to tune up properly. 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

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 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. As Alan Applegate (K0BG), author of the excellent K0BG.com website on mobile HF points out, 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 ensure that conducted RF does not get into your vehicle or electronics (this areas 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

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. We were fortunate that 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 both 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. It is also critically important for safety reasons 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 the associated cabling and to properly protect the battery cables.  We used insulated cable clamps and convoluted tubing to accomplish this.

Ameritron ALS-500M Amplifier And Radio Interface

Ameritron ALS-500M Amplifier And Radio Interface

The next step was to select a location for the amplifier that enabled good air flow around the unit and to securely mount it to the floor of the truck. The spot that we choose was under the flip-up rear seat of our truck. The mounting location we used is at the corner of the seat which ensures 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 which 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.

Amplifer And Accessories Under Rear Seat

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

Power Distribution, Antenna Controller And Wattmeter Sensors

The picture above shows the layout of all of the power and accessory electronics in our installation. Note the two high current fuses which protect 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 of our accessories. We again used black convoluted tubing to protect all of the cabling and to dress up the installation.

Wattmeter Sensors And Screwdriver Antenna Controller

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 that our radio is operating on 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

Control Layout On F-150 Console

All of the controls for the Amplifier, Screwdriver Antenna Controller and the 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

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 that one keep their eyes on the road.

The Ameritron ALS-500RC unit provides a switch to enable or reset the amplifier if it should trip as well as a remote current meter which shows how much current the amplifier is drawing.

Elecraft W2 Wattmeter

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 to have when operating at night.

Voltage Monitor

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 which plugs into the cigarette lighter jack in the vehicle. The unit has an easy to ready display and it does not draw much current so it can be left plugged in when our truck is parked.

It took a little time to setup 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 instruction cover the setup steps well.

Icom HM-151 Microphone

Icom HM-151 Microphone

The Icom IC-7000 Transceiver is an excellent radio for mobile HF applications. Unfortunately, the quality of the transmit 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 a number of upgrades which included 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 some 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 what the setup sounds like on the air.

There was quite a bit of integration and working with the folks at all of 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

Scorpion Whip Quick Disconnect

We are operating across a very wide range of bands (160m – 10m) and I’ve found that it 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 and this improves 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 10m.
  • 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 well installed screwdriver antenna setup and this 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 has certainly helped in this area as well.

We hope that 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)

4 thoughts on “Mobile HF Installation – Part 4/4 (500W Amplifier, 160M and Accesories)

  1. Fred, I really enjoy all of your articles and especially have enjoyed your mobile series. It is really great to see someone who “does it right”.
    I’m curious if you have had any success on 160 mobile, especially with DX from your QTH. Also, have you tried mobile CW?

    73 – Mike KI8R

    • Hi Mike,

      I have not had much of a chance to operate on 160m yet. I did make a handful of contacts before I got the amplifier in with the longest on the Top Band to Canada and the Caribbean (out to 1,000 mi or so). Signal reports have been in the 59 range using 100W which is a good sign. I plan to do some more operating on 160m as we get more into the winter here.

      I have not yet tried CW or digital modes from the truck yet but both are “on the list”.

      – Fred (AB1OC)

  2. Fred, I really enjoyed your series of articles and the phased approach to implementation. How is the 500W amplifier performing? Did you have any new issues with noise? How much extra range are you able to achieve? I’d love to hear more.

    73, Michael KG7QEX

    • Hi Michael,

      Thank you for taking the time to read our Blog. It is a very difficult design problem to build a mobile HF amplifier as the supply voltage limit of 13-14 Volts DC makes it extremely difficult to generate high power, especially at higher frequencies. The Amp that I am using does a fair job and reaches 450W or so on 160m – 40m. Power output on 20m is closer to 350W and I tend to see around 225W on 10m. This is not a bad profile as you need the extra power the most on the 160m – 40m where limited length of most mobile HF antennas limits efficiency. I have not had an issues with noise with my setup but note the extensive work done on bonding of the vehicle (see the other Mobile HF posts). I have made some pretty long DX contacts on many of the HF bands including a 75m SSB contact to Japan from the NE USA where I live and to South Korea on 40m SSB. I have also made contacts with India (1/2 way around the world from my QTH) on 20m. Obviously, one needs decent band conditions to make these contacts from a mobile under any circumstances but I believe that the extra power definitely helped. All of this said, an amplifier installation in a vehicle is not a simple project. I would say that the amplifier and associated power system installation in my F-150 was at least as much work as the rest of the mobile HF station installation combined. I hope that his helps you.

      – Fred (AB1OC/M)

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