which is better to tune an antenna.... radio's SWR meter or separate?

That extra piece of coax is not going to have any noticeable effect because coax is shielded. When you are adjusting your SWR's you are adjusting the final leinth of your antenna.

Something I forgot to meniton is you tune your antenna in the middle of the band, I prefer channel 21 so you arent bleeding over onto 19.

The reason for this is a cb radio opperates on a Frequency range where the radio wave is aproximately 10 meters tall. As the Radio wave modulates from negative to posative the signal will travel at the same speed as the speed of light, the frequency is determined by how many times per second the signal goes from negative to posative, and the physical size of this wave form is where you get the measurement from.

Thus a "perfect" antenna will be around 10 meters tall based on what frequency you are operating on, it will be the same height as the radio wave form.

Since it is impractical to go down the road with a 10 meter tall whip hanging off your truck, you will wind up using a antena that is a perportionate size of the wave, thus on the package when you see the term 5/8 wave, it means that the antenna wire is 5 8ths of the wave form in leinth.

This is why one of the very popular designs of antenna is the fiberglass core with a copper wire wound around it. With an adjustable tip, like the Firestix II have, you can adjust your swr.

When you adjust the swr what you are doing is matching your antenna to be directly proportionate to the size of the radio wave form in the configuration you have it mounted in.

The coax is not counted in this because the coax is shielded and thus not an emitter.

 

That is kind of true, but not in a truck, here is why an 18 foot cable would be good on a BASE STATION Every electronic device creates EMI or "Electro Magnetic Interference" EMI will mess with the effectiveness of an antenna and it will erode its ability to properly propigate a signal. The further you can put your Antenna from your radios, the less chance of EMI effecting them.

However in your truck where you have limited choice where you are sticking your radio, simply put it in there, us as short of a coax as possible and get on with life.

The reason you want to go short is it uses energy to push the signal through the coax, so the longer the coax the more loss you will have.

The reason you tune in the middle of the band is that you can only tune your radio one way, you cant tune then retune, tune it in the middle, and call it good. you arent shooting monkeys into space, you dont need to obsess over it too much.

 

Once you're tuned, you're tuned. You don't need the SWR meter anymore unless you change equipment.

 

Couple of different reasons for this:

First your radio's meter as already noted isn't going to be the most accurate, but it will definitely give you a ball park. They're also good to have for monitoring purposes. A hit on the antenna, rain etc can all cause damage and create issues with the SWR. Having the radio meter or an external meter in-line can signal there are problems before things get too far out of whack. If it's a high power radio or you are running an amp, this becomes even more important.

Second, to properly tune an antenna you should really have access to an analyzer and measure the reflect (SWR) at the antenna with the shortest piece of coax (<12") you can find. You want to see what the match at the antenna mount is first and foremost and work on the mounting location and grounding for the lowest possible reflect. At that point you add your coax and depending on what school of thought you come from you either; run the shortest run you need to make the connection or you adjust the length to give you the closet reading to a 50ohm match at the radio. I'm of the latter school and try to keep my runs 1/2 a wave-length depending on the coax size and present the best possible match to the radio.

In your truck you really, REALLY, want the lowest possible SWR reading you can get at both the mount and the radio - here’s why...

Big rigs are extremely difficult to get an efficient antenna to tune properly on. No sheet metal under the antenna and poor grounding typically result in very high SWR at the mount. Antenna designers being pretty smart individuals, know if you make an antenna with a lot of coils in it, those coil losses will give the impression of a good tune. This good looking match (low SWR) is very misleading when you're dealing with any antenna that has a coil (Monkey Made, Pred 10k, Firestick etc). If you add high SWR onto a lossy antenna, you are loosing a lot of transmit power... A lot. If you're using any antenna that is designed for a truck, you really want to be seeing 1.2 or 1.1:1 at the mount (if possible) and at the radio.

 

Corneileous,

What you're referring to there is a type of balun known as a common-mode-choke... they don't work very well in your application. The choke needs to be at the feed point (the mount) so you would have this multiple loop of coax hanging under your mirror. Additionally the chokes don't improve the match at the antenna mount, which is actually the most important. They can reduce RFI but again, not very practicle blowing around in the breeze...

 

Let me try to work through each one of these, if I miss something let me know... keep in mind, in the world of radio, like any science, there are always competing ideas and schools of thought. Guys who have been in the field for decades and can build amplifier circuits in their sleep will never agree 100&#37; with each other on how to do it. Take my input as a "potential" solution and listen to what others say as well. Then, to the best of your ability and equipment, test those ideas and find what works best for you. Of course understanding what is "best" also requires a little bit of basic knowledge - which is why your asking questions!

There are a couple schools of thought when it comes to coax length. Neither really supports the use of 18ft of coax... But the industry "standardized" 18ft because it "theoretically" equals a have wave length. [11 meters, our bands wave-length, converted into feet is 36.0892. Divide that by two and you end up with 18ft... there is a problem with the calculation though that we'll hit in just a second.]
So the two trains of thought are this...

Coax length is of no consequence especially in a mobile application where signal/power loss (db attenuation) is minimal because the length between the radio and the antenna is so short. There is some truth to the statement. I'd add this...


Coax length is of no consequence when:

1. All other systems are optimal (near-perfect match at your mount and a true quarter wave antenna with NO tuning coil is used) and/or
2. Maximum output/efficiency isn't required as it might be when shooting skip and or being involved in a keydown (or a friendly comp inside the parking lot with the "big radio" next to you...)
   The second opinion and the one I typically subscribe to is, coax length does matter because we are almost never in a perfect setup and I want to minimize every loss I can. Additionally I want to present my Radio and my amp with the best match possible, even if the match at the mount isn't perfect, keeping the hardware running cool is important to me... I talk a lot of skip and I talk for hours. If you've ever been through Phoenix, you might already know I talk for hours... and hours.

The basis for that belief is, coax can and does act as an impedance transformer - not a belief held by everyone and something that seems easily proven and dis-proven depending on your testing method. This is more than you ever wanted to know, but it will help you build that basis for your own research and understanding if you take your hobby to the next level.

Movin' on. My experience, especially in mobile applications is, I've always found using half a wave length jumpers gives me the best match. "Match" here means, my antenna is "Resonant" on the frequency I'm transmitting on AND presents a proper load to the transmitting device (our radio, and possibly our amp) which in this case is 50 Ohms.

When either the impedance (our 50 ohm load) or our antenna's resonant characteristics are off, it affects the SWR. Why is that important to understand? Because your normal SWR meter won't tell you which one it is... So you never really know if it's the length of the antenna or the grounding at the mount, or the location of the mount. This is why we use an analyzer if one is available. But even that's not a perfect solution. ANY antenna that uses a coil will show a better impedance match than is really there due to coil loss. That doesn't change the fact that power out is being lost and the radio's ability to transmit to it's full potential is being minimized. And before we jump into the rading debate about coil antenna's I'll say this: on this frequency (27mhz/11 meter) coil losses are not as severe as they are on other freq's. A three wrap Monkey Made or Mr. Coily will show 95% of the field strength that a non coil quarter wave antenna will show. However, the more coils you add the more loss you encounter... If it takes four, five, six or more wraps to get an antenna to tune it comes with a price in efficiency and the power you're able to transmit.

Now this is where people typically throw their hands up and say screw it; and for the most part you can do that and still enjoy your radio. But it doesn't hurt to understand the "why" .

So whats the problem with 18ft of coax... yeah I'm finally getting to it. Well when you adjust the coax for it's velocity factor, 18ft is typically too long. As an example, 18ft of RG-8 has a velocity factor of .66 so we need to adjust that half wave-length by that amount; in this case (and many) 12ft is actually closer to a half wave-length.

Does it matter? Again only you can decide; it will depend on what you want out of your radio and what you want to do with it. If you're just rag chewin in the lot or on the road, probably not a big deal. If you feel the need to run a high power radio or amplifier, it makes sense to get your antenna system in order first, both for efficiencies sake and the health of your equipment. Keep this in mind. If your SWR is currently 1.5:1 the amount of power coming back to your box and or radio is completely different at ever power level you move up to. 100 watts out will present a completely different level of power back to your equipment than will 1000 watts out. The more power you are running, the better and more efficient you need that antenna system to be to keep things alive.

When it comes to coax, at the end of the day, if all you have available to you is 18ft, then that's what you buy and you make it work. If you have to wind it up into a ball and zip tie it to keep it out of the way, do it. Baluns and winding do not affect power out (IMHO), only currents flowing back down the coax.
If you have the ability to get your hands on proper lengths based on velocity factor, or you can solder your own connectors on, then do it.


So tuning your antenna and what channels to use... 1, 19 and 40. Tune your antenna for the lowest possible reading on 1 and 40. This will typically give you an even lower reading on 19. Most antenna's tune in a "V" with the SWR reading being higher on the outer portions of the band. This will be affected by the size/width of the antenna and how broad banded it is. If you are using a steel whip, your SWR will typically be higher on the edges than in the middle. If you use tubing (which you would never do on a rig) than the reading will be flatter across the entire band. Antenna's like the Monkey Made and Mr. Coily are broad banded enough to be used on 10 meter as well.
If all you do is talk skip on channel six and you run a lot of power... tune your antenna to give you the best reading on that channel. Basically work for a good match where you use your radio the most. This is especially true if you are using power.



The antenna with the grounding wire was probably a wilson... A lot of people think its nothing more than a trick and I tend to believe that. If the antenna mount isn't properly grounded you can often improve that by running a ground strap but length and type are important. The shortest flat braid you can is the best. Think under 1 foot if possible, any longer and run the risk of turning the braid into part of the antenna and making things worse. Outside of that you would need some type of matching network and a wire hanging out of an antenna doesn't really fit that theory.
So... decide what you want from your radio and setup. Tune it the best you can with the equipment you have. If you want better performance think about getting your hands on these three items.



A bird 43 meter.
An MFJ antenna analyzer, and
A quality Field Strength meter.
These will help you note the impacts of the changes you make and ultimately determine the most efficient setup for your application.



Regarding the last question... What we call bull-horning your antenna (leaning the antenna forward). This changes the plane that your antenna radiates on - the polarization of the antenna. If the majority of guys you are talking to are running vertical antennas, you're going to suffer about a 15db loss between them and yourself (it's a lot). There is a 30 db loss between vertical and horizontal polarizations, this is why vertical antennas have such a hard time talking to flat beams on ground wave. Not always the case in DX land where the signals can be bounced back and twisted around. I know it looks cool and it might show you "better" numbers, but the angle losses will far exceed any improvement in match you might gain.



Sorry for the length of all this... that was brutal I'm sure!