HF Mobile 4S7AB/M
Let’s go Mobile in Paradise Island
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Ham Radio has become a diverse hobby simply because of it’s own horizons. CW to Digital communications, Terrestrial VHF to Satellite communications, Simple rag chewing to contesting are only a few areas to name. Mobile Ham Radio is another interesting facet of this wonderful hobby enjoyed by thousands world over.
4S7AB / KA5MAL
Author Oct 2009
Having More than 35 years of experience in the Field of Amateur Radio, I have published many articles in prestigious international publications like, QST, CQ-DL, NCJ, Six-News etc.. Here in this web site, these write-ups are intended to give you knowledge about my designs and projects related to the hobby. The articles about foreign HAM activities are intended to share my experience.
You don’t have to miss the early morning Long path opening to USA on 20m anymore, you can try to work them while driving to office. Getting stuck in a 1 hour long traffic jam is going to be fun hereafter because you can go on HF. You can go to the next JOTA in Hambantota while talking to the scouts on your way. You don’t have to get frustrated anymore in the parking lot while your XYL takes her own sweet time in shopping. During the next trip to the ‘world’s end’ you might want to wait at the Horton Planes and do some rag chewing with Indians on 40m while others take that 9km long walk. Welcome to the world of HF mobile.
Mobile HF operators have their own challenges and techniques. Often the vehicle body is not sufficient enough as a good RF ground for the lower bands from 30m and below. Modern day fiberglass automobile body parts make the situation worse. Therefore the operators will have to find ways to overcome these problems with compromised solutions. The other main issue is the heavy losses and hence remarkable inefficiencies in the vertical antennas with linear loading. Unless the operator is crazy enough to fix a 3-el yagi on the automobile, large percentage of hams survive with loaded verticals.
The mobile HF antennas evolved in a very nice fashion over the years. Initial antennas were fixed for a single band as a ¼ wave whip or a vertical with a fixed loading coil and sometimes with a top loading capacitance hat. Then the verticals came with a tapping for each band. But the problem was that the operators had to stop the vehicle and manually change the tapping for changing the bands. To overcome this situation, the motorized antennas came with the possibility of adjusting the amount of linear loading through a press of a button. The screwdriver antenna is one such commercially available solution for mobile operators. The modern mobile rigs can automatically adjust the antenna once the desired band is selected.
Another issue faced by the mobile operators is the ignition noise both in the TX and RX signals. DC filtering can be employed at various levels of the DC path to overcome the problem. Modern rigs have DSP based Noise Blankers which are very effective in fighting this problem.
Fast fading is very common in HF mobile signals since the RF ground conditions and terrain change from place to place while moving. But all of a sudden, you can see the signals peaking 20dB over S9 while passing a river, waterfront or a wet paddy field. Similarly, the RX signals get bombarded with RF hash when passing industrial areas.
Present day hams living in the urban areas have a challenge of limited space. Mobile operation is a nice solution for such hams all over the world. They can ride to an open space, mountaintop or to a beautiful beachfront and enjoy the hobby at leisure. Some extra gain can be obtained by throwing some radials to the water if needed. Further the hash and all QRN can be avoided when you go away from the town. Such mobile operators are heard on all bands and it is not very uncommon for them to get a ‘priority’ in a pileup.
There is HF nets and contests dedicated to mobile operators around the world if you are a contest chaser. Some major contests have a mobile category to encourage mobile operations. If not, HF mobile can give you continuous coverage independent of the VHF repeater coverage. This is also a very handy way to be prepared for disaster relief operations. It is always advisable to jump into such an operation with a previously tested setup.
-by 4S7AB 2006 (This article was published in April 2006 RSSL News)
Very First HF-Mobile HAM Radio Setup in Sri Lanka
My HF Mobile Setup after 2009. FT857D inside the Jeep and ATAS-120A mounted on the Spare-wheel (more pictures below)
In Sri Lanka, HF mobile operations were restricted until recently. I was able to get the first HF mobile license on the 6th of April 2006, after a seven-month waiting period. Everyone who can fix a HF mobile setup is encouraged to apply for the license and join this fascinating side of our hobby. I have completed my first 3 QSOs with Colombo based stations 4S7EA, 4S7WN and 4S7AA respectively on the 7th of April 2006 on 40m getting 57 while ‘mobiling’ from Attanagalla to Nittambuwa in my home area. Special thanks goes to the present and past office bearers who have pushed the authorities for obtaining the HF mobile licenses for ham operators.
I hope to see many more stations coming to this exciting area of our hobby.
Below are some photoes with the Mobile setup installed in my Pajero 4×4 Jeep