Wednesday, July 08, 2015

HF Mobile Antenna Installs

If you have recently installed an HF rig in your mobile along with an antenna or are planning to, it is important that you realize that the antenna is only one half of a dipole, and in the case of a mobile antenna the missing half is the vehicle and its capacitance to the ground under the vehicle.
The single biggest factor with respect to efficiency for any mobile and especially on 40 and 80 meters is ground loss, so maximizing this half of the antenna system is essential, and this is accomplished through bonding.
After finishing the install, you possibly have found out that you are experiencing engine noise on the receive side of your transceiver.
You will find that you can eliminate a lot of the noise with the use of ground straps to bond the various panels that make up the body of your mobile.
Besides the various body panels that you will want to bond, the biggest culprit of noise generation in a vehicle may be the exhaust system.
The majority of vehicles have injectors and spark plug coils in place for each cylinder. These devices can create substantial pulses on the vehicle wiring acting against the ground system. Engine blocks often have a poor ground, so anything attached to them can radiate noise. The exhaust system is directly bolted to the engine and can behave just like an antenna, radiating noise.
After initially installing the HF gear in my mobile, I found that I was experiencing some engine noise.
  I was sure that grounding the exhaust system would rid me of the some of my problem with noise, and this proved to be the source of the noise that I was experiencing in the receive side of my HF rig.
 I installed a U-clamp around the exhaust-pipe at the rear of my mobile, and then fastened a flat-braid ground strap between the U-Clamp and the frame. I installed another U-Clamp and ground strap at the engine end of the exhaust system as well. This proved to be where the noise was being generated, and I now have a noise free receiver.
For any bonding you do to your mobile, it is important that you use flat braided straps and not round wire for bonding, as RF travels on the surface rather than through the wire, therefore flat braid has
more current carrying capacity due to its larger flat surface.
Another way although not as simple to install, is the use of wide flat copper flashing like a friend of mine used for bonding his mobile.
It is important that you keep all of the ground straps as short as possible, and if you cannot use existing body fasteners, do take care when drilling holes in the body, so that you do not drill in-to any existing electrical harnesses that may not be visible.
 I used quality stainless steel fasteners along with stainless steel serrated washers for attaching the ground straps throughout my mobile.

Something else I will discuss in regards to the install of an HF antenna on your mobile, is the fact that no matter how carefully you have planned the install of the antenna, and the route you took to run the coax from the antenna to the transceiver, you will more than likely suffer from common mode currents travelling on the outer shield of the coax. This is especially a problem with poorly mounted antennas, whether it be through the mounting of your favorite antenna like the very popular Little Tarheel screwdriver or the Yaesu Atas 120 antenna on a lip-mount fastened to the trunk lid with set-screws, or up front of the vehicle with a L-bracket fastened to the inner fender with sheet-metal screws. As the set screws or the sheet metal screws loosen off due to movement of the antenna, its a given that you will suffer excessive ground losses leading to RFI problems.

Even if you take the time to design and build a very good mount that is capable of with-standing the stress placed upon it by your HF antenna and is very securely mounted, it is in your best interest to add a choke at the feed-point of the antenna.
You can easily accomplish this through the use of a mix-31 ferrite split-bead placed over the coax preventing RF from flowing on the outside of the shield.
Mix-31 ferrite beads have a high permeability and work best for frequencies below 30 MHz as opposed to mix-43 ferrite beads that have a lower permeability and work best at frequencies above 30 MHz. I don't have the space here to discuss how ferrite works, so I'll save that topic for another day, but what ferrite will do for our application is to stop common mode currents from flowing on the shield causing RFI issues, but allowing current to flow through the center conductor as required.

To assemble the choke you need to coil 6 turns approximately five to six inch's in the coax at the feedpoint of the antenna, and then snap a Mix-31 split-bead over the 6 turns of coax.
Now you may be asking where you can purchase ferrite split-beads and what size you require to build this choke. I have found DX Engineering to be a good source of ferrite split-beads. You will want to purchase Mix-31 split-beads with a 3/4" ID. This size will nicely snap over 6 turns of RG-8X coax.
Remember a split bead snapped over a single run of coax is useless, you need to snap one split bead over 6 turns of coax for the choke to be effective in stopping common mode currents from flowing on the shield.
You will want to use the same type mix 31 split beads for the screwdriver motor control leads as well, as this is another area where common mode currents can flow on the power leads causing all kinds of RFI issues at your radio stack that includes causing damage to the digital screwdriver controller you may be using. Wind 6 tightly wound turns of the power cable through and around a single mix-31 split bead.  This bead can be of a smaller diameter that the 3/4" bead you used for the coax. I used a bead with a 5/8" ID on my mobile for this purpose. Do not just wind the power cable through the split bead haphazardly, but keep the turns wound side by side evenly through and around the split bead for maximum effectiveness in stopping RFI issues.
So that's all for now, and if you want to learn more about going HF mobile, visit my website here....

Note...all the photos expand by clicking on them.

No comments yet