Dual Antenna Motorcycles


Compliments of Firestik® Antenna Company Technical Support Team

Copyright © 1996 Firestik® Antenna Company

Over the years we have worked with many motorcycle owners that were dissatisfied with their factory set-up. As the word travels that we are willing to work with the road-bike owners, we are finding that more and more riders are coming to us hoping to improve their communications system. The amount of traffic on the topic has grown to the point that it has become necessary to write this document. Since we seem to be answering the same questions over and over again, we are hoping that this at least fulfills 80% of them. Besides the universal information regarding basic antenna selections and installations, this document answer many of the installation questions you have. Because there are several. if not dozens of installation variations, this will be one of the longer tech documents that we've written. You will need to sort through all of this and take that which may relevant to your particular situation. Also, keep in mind that this is an on-going process of learning. Much of what we've learned, we've learned from working with users such as yourself. If you have some insight based upon your own experiences, please drop us an e-mail. Some of the most useful information we receive comes from non-technical sources. You are the "real world" that we need feedback from

Motorcycles that are using dual antennas, one for CB and the other for AM/FM radio, will never get optimum performance from the CB radio. This may not be a problem for most riders. As they say the proof is in the pudding. If you are using dual antennas and the set up is giving you everything that you need or expect then the technical flaws, with one exception, are not relevant. You should know what the SWR readings are on your CB set up. Why? Because if the SWR is very high 3.0:1 or higher there is a possibility that you will burn up the output transistor within the CB. High SWR creates heat in the CB. While you may get away with high SWR if your transmit duration's are short, there isn't any specific time that can be labeled as safe. Some transmitters seem to be more tolerant then others. Those that aren't just go poof!

The close proximity of the AM/FM antenna to the CB antenna on motorcycles allows it to absorbs approximately 30% of the CB antennas radiated power when in transmit mode. It will also distort the radiation pattern because its magnetic properties will interfere with the magnetic waves being emitted from the CB antenna. But as mentioned above, if you have acceptable SWR and are getting what you expect you will probably lack the need to read any further. Just go out and have fun! The rest of you can keep reading.

Some motorcycles have an inherent problem with ground plane (counterpoise) simply because there just isn't enough metal to work with. We have found that most riders trust that the antenna system installed by the manufacturer is "ret-ta-go". Very few understand the need to have the antenna system (the coax, the mount, the antennas and the motorcycle itself.) SWR checked. With the cost of motorcycle radios being 2-3 times more than those for cars, we are thinking that there should be more interest in the SWR matter.

What are riders doing lately? We'll start with the easy ones first. In some manner or another, some have bikes that either came without the communications system installed (maybe that was an option), bought it used without a system, or hated the way it worked and yanked the CB out. Nonetheless, there wasn't anything old to deal with in these cases. The majority of these owners went with a NGP system. Some of them would mount either the DS14-FG or the FG2-DD kit on top of a storage box lid. Others with more fabrication spunk would make a bracket and the rest would find a place and way to use one of our universal brackets (M2 or SS-64). If they made their own bracket they would purchase a NGP antenna, a NGP coax and a K-4 stud mount. This would work on any bracket with a 1/2" (26mm) hole. With this set-up everything is compatible. Some users might add a K-1A quick disconnect to facilitate the easy removal of the antenna or use a DV-90 fold-down accessory so they could lay the antenna down when a cover is installed. If the bike also has an AMFM radio, our AR-1 splitter could be used to allow the one antenna to serve both radios.

The next group was not too concerned with the two-antenna look. They would completely remove the AM/FM antenna and replace the stock CB antenna with a Firestik II or FireFly antenna. They would also use a splitter to feed the two radios from the one antenna. These people stuck with the ground plane dependent antennas and used the existing mount and coax. The obstacle with this approach involved the difference in thread dimension between the stock antenna (8mm x 1.25 pitch) and the standard American antenna threads (3/8"-24). Maybe they had a lathe or knew somebody with one as they would make or have made an adapter with an 8mm male thread and a 3/8"-24 female thread (needs to be at least 0.600" deep). One person (that we know of) took one of our quick disconnects (K-1A) and re-threaded it to 8mm (for reference purposes, 3/8" = 0.375 and 8mm = 0.315). While we do not have any specific SWR data on this set-up, we have not logged and tech calls that indicated a problem. Inasmuch as the stock antenna was a ground plane dependent center load it stands to reason, from a theoretical point, that using a top loaded antenna without having a power grabbing, pattern distorting AM/FM antenna in its near field of radiation that the performance would be enhanced. It is very likely that there would be some connector issues with some of these installs. Some of these communications systems used AM/FM pin plugs for input on the CB and the AM/FM radio. The standard splitter has a PL-259 input for the antenna and a PL-259 output for the CB and a pin plug for the AM/FM radio. Replacement connectors are openly available for those with the desire or experience to process coax (not that hard). The length of the output leads on the AR-1A are not critical so either or both can be reprocessed as required.

And now we move into the "won't budge from the dual antenna look" group. The first sub-section of this group were those people who just simply changed both of the stock antennas with a matching pair of Firestik II's or FireFly's. This decision avoided the need for a splitter but still involved the need for some type of thread adaptation ... either a special adapter or the re-working of male end of one of our standard accessories (typically a spring or quick disconnect). This group would use any antenna from 2-foot to 4-foot in length depending on their personal preference. Nearly all of these were "FS' or "FL" series antennas. With this set up you will still have the absorption and distortion conditions. You will however gain antenna efficiency as a result of switching to a more efficiently designed antenna.

And finally, there was the one guy who didn't want the inherent problems of having the two antennas so close together but would not turn his back on the dual antenna look. We ended up sending him a splitter and one Firestik II antenna wound with wire that he used for CB and AM/FM and a second one we wrapped with string to look just like the real antenna. Without having the wire on the dummy antenna we eliminated (or greatly reduced) the absorption/distortion problem. He had to deal with a few issues involving connectors and adapters but after he was all done he called to tell us how great the set-up worked.

One of the problems you may run into might be one of bandwidth. That is, you can get good SWR in the center of the 40 channels (frequency wise this is channel 19) but when you get out at the ends (channels 1 & 40) the SWR gets too high for comfort. This is a problem often experienced with physically short antennas as well as on installations with ground plane problems. If that occurs, keep in mind that you can shift that best SWR to another location within the band. As an example, I run a short antenna on my truck for nothing more than the convenience of wheeling in and out of my garage. Between the size of the antenna and the location where I mounted it, I cannot maintain sub 2.0:1 SWR across 40 channels. Since I never use channels above 19 I shifted the SWR dip point from channel 19 to channel 10. It is easy to do! Instead of taking SWR readings on channels 1 and 40 and making antenna adjustments to make those readings balance (that procedure makes channel 19 the optimal center), you take readings on channels 1 and 19 and make adjustments that balance the SWR between those two extremes. On my install, that shifted the resonant point to around channel 10 and offered me sub 2.0:1 SWR across the channels that I use. It doesn't matter what two channels you use for reference the tuning procedure is the same. That is measure the SWR on the highest channel you use and the lowest channel you use. With that information, make adjustments (tuning screw down if SWR is higher on the highest number channel or up if it is highest on the lowest channel you use) and that will put the SWR dip right smack in the middle. So review the channels that you use and if it is less than all 40 channels, shift your lowest SWR point (dip) to the center and that will optimize your performance on those channels.

So there you have it at least for now. This will probably be a live document that will change as we continue to get feedback from users. There are four groups of communications people that are really innovated the hardcore hobbyist, the Jeep owners, the RV owners and you road warriors. Okay isn't good enough and as frustrating as it can be, performance enhancement is a challenge you all seem to pursue. We enjoy the process but must admit that there are just way too many possibilities to be specific when it comes to these installations. Our best bet is to be a facilitator for all of you and to that end, hope that this offers some help. So if you have something to say say it and e-mail it to us. We will monitor the feedback we get to determine if there is a great enough need (and market to justify the investment) for us to look into adding some specific accessories to help you make your installation or conversion easier or more convenient. We have been market trained to think universal because the CB market just isn't big enough to justify a specific system for every vehicle on the road. In the last few years we have made some exceptions with specific mounts for Jeeps and Dodge Ram trucks so ... there is always hope for some bike things too.

Firestik Antenna Company - 2614 E Adams St - Phoenix, AZ 85034 - (Tel) 602-273-7151 - www.firestik.com

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