Many amateurs want to take their hobby mobile. Professionals do mobile operation all the time, but they have professionals to do the installation for them. This technical note provides advice on how to do a worry-free mobile installation. To the left K8SWD, WD8AXA and K8WPI show off their mobile installations using the SGC QMS.
More information on mobile installations is available in SGCs HF SSB Users Guide or in our book Go Mobile at 500 Watts. Both can be downloaded in PDF format from our publications page.
Installing a mobile transceiver is not magic. The physics don’t change just because it’s a car or truck. The principles remain the same as in your base station. Here are some basic guidlines to help you through the process:
1. Locate your antenna correctly. Aesthetics and RF are sometimes at odds with one another. It may look nice to install an antenna on a bumper or you may find it easier, but it will definitely hurt your signal. You can lose as much as 3 dB on a passenger car and up to 9 dB on a van with a bumper installation.
2. Place the antenna as high as possible. Any antenna should be as high and as clear as possible. On a vehicle, this can be hard to do, particularly at lower frequencies. On a van, the best place is usually the left or right rear of the roof. GMs recommendations in their “Radio Telephone/Mobile Radio Installation Guidelines” suggest that antennas should be in the center of the roof or the center of the rear deck on passenger vehicles. This is generally impractical for HF antennas.
3. Don’t use a pickup toolbox. Metal toolboxes on pickup trucks are really tempting mounting points, but they’re almost always going to be worse than using either the center of the roof of the cab or the rear corner of the truck bed.
4. Use a resonant antenna, a coupler, or both. If you are going to operate at a specific frequency or within a very narrow band around that frequency (within 20 kHz or so), then a properly resonant antenna is always your best choice. Mobile operations don’t usually give us that option. We usually want to be frequency agile when working mobile, so we want to use the antenna across a broad band of frequencies. A coupler, such as an SGC Smartuner™ is your best choice for making this work.
5. Don’t trust a built in tuner. Any tuner at the radio end of the coax is trimming the cable to match the requirements of the transceiver. In many cases, this is good enough but in a mobile installation it can really restrict your operation. A built-in tuner does nothing for the coax between the antenna and the tuner. In fact, we KNOW that SWR is likely to be high because except at very specific frequencies, the coax will not match the antenna feed point impedance. You may be able to load the antenna and get an SWR indication at the transceiver less than 2:1, but your feedline will not be under control and this can cause losses in your signal. You may think you’re putting out full power, but you won’t be radiating it.
6. Locate the coupler at the antenna feed point. The optimum place to locate an antenna coupler is within 6-8 inches of the antenna feed point. The longer the wire is to the feed point, the higher your losses will be. If the lead wire is 1 foot long to a 8 foot whip antenna, then 11% of your antenna is inside the car! Up to 1/4 of your radiated power can be lost in this situation.
7. Mount the coupler outside the vehicle. If at all possible, mounting the coupler outside the vehicle will reduce noise pickup and improve your operation, otherwise the body of the car will act like a Faraday shield . This is one of the reasons why SGC developed the QMS Quick Mount System.
8. When stopped, improve your antenna and ground. Every mobile installation meant for operation while moving is a compromise. Antenna lengths are not chosen for maximum radiation, but instead are limited by the practical requirements of the vehicle and the air moving by it. We simply can’t mount an 80 meter dipole on a car when we’re moving. BUT, when we stop, we can improve our antenna and ground with some simple steps:
Add length to your antenna – bring along 50 feet or so of #12 wire and with an alligator clip on one end. Attach the clip to the end of the whip and string the rest out to as high a tree as you can or to a pole that you bring along yourself.
Attach a different antenna – bring along a different type of antenna with a connection that can screw into your existing antenna mount. This could be a larger vertical or some other type of antenna to improve your signal.Improve your RF Ground – battery jumper cables make for great RF Ground connections.
Ground your vehicle to a fence, grounded water pipes, or lawn sprinklers. It will make a world of difference in your signal. You can also carry an extra 50 feet or so of #12 wire, the same as mentioned above to lengthen the antenna and attach it to the RF Ground connection on the coupler as a counterpoise.
Screwdriver antennas work well for mobile installations, but they really don’t allow you to improve your operation when you’re stopped. A good antenna coupler will allow you to add length to the antenna and add additional grounding when it becomes available wherever you are.
9. Use a remote head radio. A remote control head radio lets you mount the radio near to the antenna coupler. This will reduce feed line losses to nearly zero and reduce susceptibility to vehicle noise. Radio feed lines are very susceptible to noise, but audio lines and control lines are usually not. A remote control head transceiver will thus improve your signal to noise ratio, but with the added benefit of reducing clutter in the passenger compartment.
10. Use large gauge power leads. Most manufacturers supply #12 wire for the power leads to the radio, but to get maximum performance, a heavier wire such as #6 or #8 might be a better choice. A modern HF SSB radio with 110 watts of output will require about 240 watts input, which translates to 20 amps at 12 VDC. Transceivers running up to 150 watts, which is common with commercial gear, may draw over 30 amps at peak loading. AWG #12 wire just isn’t enough to handle these loads. Power leads for your transceiver should go directly to the vehicle battery or your auxiliary battery. Both leads should be fused, even the ground lead. If the ground strap to the engine were to break, then the vehicle would attempt to pull starter current through the only ground connection available, your transceiver.
11. Bond your vehicle. Most people think that a single ground connection gets the whole car involved in your RF Ground. That just isn’t the case. Modern assembly techniques use spot welding for assembly of unitized bodies. Many of the metal parts used are soaked in paint for rust protection. Remember, the manufacturer is more concerned with controlling rust than improving RF! To get the most our of your vehicle’s RF ground potential, you need to bond all doors, the hood, the trunk lid, in fact everything that opens. Even the frame itself may not be well bonded together as an RF Ground.
In addition to providing an RF Ground, unbonded components can be sources of noise. We were once asked about why we were bonding a tailpipe on a jeep. Well, the tailpipe was rusty and the connection from it to the engine was just right to create noise when used in dusty terrain. The thumbrule here is “When in doubt, ground it out!”
Inspect your connections regularly. A mobile environment is hard on any sort of electrical connections. Your RF ground and your antenna need to be inspected regularly because the Smartuner will hide slow changes in your antenna or ground system until it can no longer compensate for them. You may operate for a long time as your fittings corrode and then find that you can’t operate at all. It will seem sudden, but the problem grows gradually.
12. Double your grounds and ground bolts. When securing braid to your vehicle, make sure that you scrape the metal clean of all paint and sealants, then install 2 ground bolts, not just one. This may appear to be a minor point, but even the smallest resistance at the RF ground can cause sporadic tuning, lower received signals, and other problems. Doubling up is also useful for the RF ground connection itself. Instead of running a single braid, run more than one. At least run braid to the chassis and to the body. Get as much metal as you can into the RF ground.
Keep ground straps as short as possible. Connecting to your RF ground can be tricky. Often people will use a Volt-Ohmmeter to check their ground straps and declare them good because there is little or no resistance. However, the ground strap is not for DC current. An RF ground is carrying RF energy and a DC resistance to ground will not show if there is an impedance to ground at RF frequencies. Be aware that RF conductivity is not the same as DC conductivity.
One of the great misunderstandings in checking grounds is that just because you measure continuity with a VOM, that you will have a good RF ground. You MAY have a good RF ground, but a DC measurement doesn’t prove it. Remember, if you measure the resistance across an inductor, it will show zero DC resistance to ground even though it’s impedance at HF frequencies may be quite high. Continuity is a good check, but it does not certify your RF ground system.
A useful test of the quality of your ground is to lay out several long wires while you are stopped in your driveway. Connect them to the RF ground connection on your Smartuner. When you remove these temporary wires, reconnect your grounding system. The signal should get better. If it gets worse, your RF grounding system needs work.
13. Add a second battery. One of the best ways to assure adequate power for your radio is to install a second battery near the radio. With appropriate provisions for keeping the battery fully charged, this will ensure you have adequate power, isolate you from some forms of conducted noise, and prevent accidentally draining your vehicle’s battery.
14. Track down noise sources. Noise in and around vehicles has only gotten worse over time. A cheap handheld AM radio can help you identify sources of noise. Set the radio for the high end of the AM band and turn the volume up. Now move around the car to identify where sources of noise are located. Noise volume should go up when you are near a hot noise source. One of the sources of noise in many cars, particularly older ones, is ignition wiring that needs to be changed. Old ignition wiring needs to be replaced regularly or it becomes a noise generator. Newer cars have microprocessors everywhere and these are broadband sources of noise.
15. Shield ignition wires carefully. Shielding ignition wiring is an art form. A bypass capacitor slows the rise time of the ignition pulse and will keep it from rising as high. This means that the timing may be retarded and the spark won’t be as hot. On many modern vehicles, this is simply not an option any more unless you have access to sophisticated equipment that can properly adjust the engine so as to keep it running correctly. Changing timing and reducing the spark can affect emission controls as well and can lead to licensing issues in some states. As a general rule, the more shielding around the ignition wiring the better from an RF noise perspective, but the better the radio performs, the worse the car will perform.
16. Install bypass capacitors. Cars coming off the assembly line are built for driving, not DXing. This means that a lot of car wiring is such that clicks and pops occur on just about any reasonable receiver. The introduction of microprocessors with their high frequency square wave clocks has only made the problem worse. Just about any accessory can be a source of problems: Thermostatically controlled accessories such as cooling fans or air conditioners are common sources. Turn signals, generator/alternator leads, brake lights, heater fans, power window or mirror motors, and practically anything else electrical or electronic within the vehicle is a source of interference.
17. Buy a diesel powered vehicle. This may sound a bit drastic, but the logic is perfect. If ignition noise is a problem and prevents you from mobile operation, then getting rid of the ignition system will help create a more noise free environment.
Finding more help. There is a considerable amount of useful information available through the ARRL Technical Information Service pages related to mobile installations. Another useful book is the ARRL “Radio Frequency Interference” manual.
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