Presentation On Building And Operating A Mobile HF Station
We recently had the chance to do a presentation on building and operating a mobile HF station for the Nashua Area Radio Club here in New Hampshire, USA. I thought it would be interesting for our readers to see this presentation as it contains some new information that we have not previously covered on our Blog.
Mobile HF Antennas
Safety in mounting mobile antennas and anything else on the exterior of you vehicle is a primary concern. This was discussed in some detail during the presentation. The best source to understand safety considerations and proper installation and mounting of Mobile HF antennas is Alan Applegate’s excellent website, K0BG.com.
The most important part of any Amateur Radio Station is the antenna system. This is especially true in a Mobile HF Station because antennas in these applications are almost always short loaded verticals. To create an effective antenna system for a Mobile HF application, one must pay extra attention to the “3 R’s” – Radiation Resistance, Loading Coil Loss, and Ground Loss. Radiation Resistance (a measure of the antenna’s ability to transfer transmitter power to radiated waves) is the “good R” and the other two R’s disipate power from our transmitter in the form of heat.
Mobile Antenna System Typical Parameters
There is some good information on the typical efficiency in the ARRL Antenna Book. As you can see from the table above, the Radiation Resistance of a mobile antenna becomes quite small on the lower bands (40m, 80m and 160m). Also note that as the antenna becomes increasingly shorter to RF on these bands, more loading coil inductance is needed to make up for the short radiator length on these bands. On these bands, Coil Loss and Ground Loss can easily dissipate most of our transmit power in a very inefficient antenna system. The net of all this is that one must pay careful attention to controlling the Ground and Coil losses while trying to make the Radiation Resistance of the antenna as high as possible. One good way to improve the Radiation Resistance of a mobile antenna is to make the whip longer. For more on mobile HF antenna efficiency, please consult K0BG.com.
Scorpion SA-680 Screwdriver Antenna With Rod And Cap Hat
Here in New England, we have many low tree branches which limit the practical length of a mobile whip. A good technique, if the installation permits it, is to use top loading in the form of a Capacitance Hat. The Cap Hat makes the antenna appear longer and thus increases the Radiation Resistance of the Rod below it. The increase in apparent electric length at Radio Frequencies also means that less loading coil inductance will be required which in turn also means that the Coil Loss is lowered. This is a win-win. The only problem is that this sort of setup significantly increases the wind load on the antenna when driving so a mechanically strong antenna and mounting system is required for a safe installation. Ground Losses can be minimized by making the vehicle on which the antenna is installed a good RF surface to couple to the ground. This is best accomplished by proper Bonding of the metal surfaces on the vehicle to each other and the vehicle’s frame is there is one.
Mobile HF Equipment
The next part of the presentation covered the selection of equipment for a Mobile HF Station. Safety and good usability are the paramount concerns here. I believe that a Transceiver should have the following attributes to be a good choice in Mobile HF applications:
- It should have at least 100W output on the HF bands
- It must have an effective Noise Blank and a good Noise Reduction system
- It should have a removable control head to facilitate mounting of the radio’s controls and display where they can be easily seen without taking one’s eyes of the road
It is extremely important to consider safety in all things mobile HF. Safe, non-distracting mounting of controls is a top concern. One also needs to consider what could happen in a crash. Loosely mounted parts or anything that can get between a deploying air bag and the vehicle’s passengers are among some of the important safety concerns. One should also consider accessories that facilitate safe mobile operation. Automated antenna controllers and a voice recorder to capture contact details for later transcription in logs are some good items to consider.
Bonding And Choking
I believe the bonding and the associated effect on noise levels and ground losses is perhaps the most important factor in determining the performance of a mobile HF station. “If you can’t hear them, you can’t work them.” Proper bonding of the exhaust system, body parts and the engine’s ground are some of the key items in this area. You can read more about how we did this here. To give some idea of how important this area is, I was able to take the initial S9+ noise levels (with the radio’s preamp off) of my F-150 pickup truck prior to proper binding to an S3-4 level with the radio’s preamp on. This is a huge improvement and is a primary reason for the DX performance of our mobile HF station. Bonding also lowers the Ground Losses of the installation which improves the efficiency of the antenna system when transmitting as well – again a win-win. Proper bonding is not expensive but it does take some work. One must also be careful when drilling holes for the installation of ground straps that you do not accidentally drill into wiring harnesses, gas tanks, electronic boxes and other vehicle systems. Again, consult K0BG.com for more information on how to properly Bond your vehicle. If you use a screwdriver antenna, you must also properly choke your control leads to keep RF out of your vehicle and its electronics. Here’s some good information explaining how to do this.
Stage 1 Mobile HF Station
I am a proponent of building a Mobile HF Station in stages from a simple one using to perhaps a more involved project later on. This allows the operator to have a lot of fun on the air with a reasonable initial amount of work and expense. The approach also provides the opportunity to see how the various steps outlined in the presentation contribute to improved performance. Our stage one installation consisted of a 100W transceiver and Hamstick antennas. You can read more about our Stage 1 installation here. The focus at this step includes proper bonding/noise control, safe installation of a suitable transceiver and simple Hamstick antennas. This stage gives you an inexpensive and effective, one band at a time, station on the 20m and higher HF bands. This type of installation is not difficult to do as is possible on most vehicles.
Stage 2 Mobile HF Station
A Stage 2 installation would probably involve a multi-band remotely controlled antenna – typically a screwdriver antenna. You can read more about our Stage 2 installation here. It’s important to choose and efficient screwdriver antenna. You can read more about the choices and what to look for here. We used a Scorpion SA-680 Screwdriver Antenna and are very happy with it. This is a big antenna and you must carefully focus on a strong and secure mounting system to use it safely. Our Stage 2 station was QRV on all HF bands from 80m – 10m and utilized a screwdriver controller to automate the adjustment of the antenna when changing frequencies and bands. We use a 4′ rod and a Cap Hat to improve the efficiency of the antenna system as well. This is an important safety feature and should be strongly considered in any screwdriver antenna installation.
Stage 3 Mobile HF Station
A Stage 3 Station is probably not for most folks due to the added complexity and cost but it does create a “work the world” Mobile HF Station and can open the door to effective operation on 160m from a Mobile Station. This step involves the installation of an Amplifier and may also include extension of the antenna system to operate on 160m. I would have to say that the upgrade to Stage 3 was as much work in our station as Stages 1 and 2 combined. It also brings a new set of important safety considerations due to the high current DC powering required by a mobile amplifier. You can read more about this Stage of our installation here.
Operating Mobile HF
Our presentation included some tips for operating a Mobile HF station. See the graphic above for details. I believe that even a well executed Stage 1 station coupled with good operating technique and some patience can yield a DXCC in many parts of the US and Europe. I was able to make many contacts to Europe and some to Australia, New Zealand, Hawaii and Alaska using our Stage one setup. If you progress through Stages 2 and 3, this gets easier. We have worked over 110 DXCCs from our Mobile HF Station and confirmed 100 in about 9 months with our Stage 3 setup. The Stage 3 setup has produced some of our most memorable QSO’s to date including my very first 75m phone contact ever to Japan and a contact with Ulleung Island, South Korea on the 40m band using SSB phone (these contacts were made from the East Coast of the USA).
Fun With Mobile HF – US Counties Tour
There are lots of fun things that you can do with a well-built mobile HF station. I have worked many DX contacts from the mobile for example and some have netted as many as 75+ DXCC’s in a weekend. County Hunting and Tours to activate rarer US Counties are another popular activity for Mobile HF operators. You can read about one such County Hunting tour here on our Blog.
We have found building and operating a Mobile HF station to be a lot of fun! It’s almost like beginning our Amateur Radio experience all over again.
– Fred (AB1OC/M)