coax myths???

[Stites]

Hi yall, haven't been on in awhile. Anywho, I've heard things about coax cables having to be 18' long to work right, that their length has to be a multiple of 3', that you need to run 75-ohm cables for duals and 50-ohm for singles, stuff like that.

Any truth to it all? Tried searching, but the search engine won't work on my phone.

 

[n5ims]

The 18' myth is part true and part false. That length isn't really critical, but due to resonance, it does help a SWR meter read a lower value (although the true SWR will not change depending on the coax length). This is because most SWR meters read voltages and this length will provide the lowest voltage at CB frequencies. Those 3' lengths are similar, tapping into lengths where lower voltages may appear.

The typical CB antenna is designed to work at a 50 ohm impedance so normally you should use 50 ohm coax. A phased array of two antennas (those trucker dualie antennas) must be phased to work correctly and specific lengths of 75 ohm coax from each antenna to the combiner will properly phase them for a front-to-back gain (with loss to the sides) and provide the radio with the desired 50 ohm match.

 

[MBill]

Stites,

You may want to go to the Firestik site. They make CB antennas and have quite a bit of
information in their "Tech Help" section, all related to CB antennas, cables, installs, etc.
I believe they cover the 18' issue, etc.

 

[jackj]

18 feet is a half wavelength at CB frequencies so a transmission line of that length will reflect back to the transmitter the exact match that appears at the antenna. So your SWR meter will read the same at the transmitter as it would read at the antenna. As N5IMS points out, a SWR bridge reads voltage and will only give accurate SWR readings when placed at half wavelength intervals along the transmission line. That is the main problem with using a SWR bridge to tune an antenna.

A better device to use is a directional watt meter. You can set it to read forward power or reflected power and the reading will be accurate no mater how long your transmission line or where in the line you insert the meter. There are several manufacturers who make directional watt meters but they are more expensive than a SWR bridge.

 

[cmdrwill]

You can get low VSWR readings even with a bad antenna by cutting the 50 ohm coax cable to the 'magic' length. Even tho the VSWR or reflected power is at a minimum, the antenna still does not work. You can even 'tune' a short circuit or even a open circuit by cutting the cable to that magic length.

IF the antenna is 'tuned' and resonant then the cable length does not make any changes.

Been there, Did that way back in the 60's,

 

[mikewazowski]

That was actually a job interview question for me many years ago.

If your VSWR is 1:1, does that mean you've got a perfect antenna system?

My reply was no, all it means is that the antenna system is presenting itself as a perfect match. Doesn'd tell you anything on how well your signal is radiating from the antenna. A dummy load should give you a good VSWR but it sure doesn't radiate very well.

 

[Stites]

Cool info, thanks yall!

I was actually going to get a Firestick before a guy at work cut me a deal on the Francis whips I have now. I'm quite happy with them, just want to get the most out of them that I can. Next step will likely be steel whips, though I'm in no real big tizzy to do it.

 

[prcguy]

The 18ft length is a myth. It might be around a half wavelength in free space at CB frequencies but that does not take the velocity factor of the coax into consideration.

If an antenna has a perfect match at the feedpoint, a half wavelength in coax will repeat that match but with velocity factors running between about .66 and .85 for most modern coax, a half wavelength in coax could land between 11.88ft and 15.3ft. Not 18ft.

If you look at any mobile antenna mount sold with coax for any frequency from 1.8Mhz to 900MHz, they just about all come with 18ft of coax. That's to reach any reasonable part of a vehicle without running too short or costing the mfr extra $$ in wasted coax. Period.

For dual CB antennas its a different story and two identical runs of 75 ohm coax that are odd multiples of a 1/4 wavelength with velocity factor included will make a power divider for the frequency of interest. Most dual CB antennas are sold with a 75 ohm coax "phasing harness" that is simply a power divider that matches the two 50 ohm antennas to the 50 ohm radio. If you want a phasing harness for a band different than CB then its dimensions will be different.
prcguy

 

[prcguy]

Did you get the job?
prcguy

That was actually a job interview question for me many years ago.

If your VSWR is 1:1, does that mean you've got a perfect antenna system?

My reply was no, all it means is that the antenna system is presenting itself as a perfect match. Doesn'd tell you anything on how well your signal is radiating from the antenna. A dummy load should give you a good VSWR but it sure doesn't radiate very well.

 

[mikewazowski]

Yup, 15 years later I'm still there.

 

[K7MEM]

If you want the straight scoop on SWR, you might try reading the PFD listed below:

http://www.k6mhe.com/files/Reflect_rev1.pdf.

It may help dispel other myths that have creeped into your heads.

 

[K5MPH]

I have seen some CB antennas come with 21 feet of coax and some with 12 feet.......

 

[SCPD]

try putting a dummy load on end of coax if you have one.

 

[Stites]

If an antenna has a perfect match at the feedpoint, a half wavelength in coax will repeat that match but with velocity factors running between about .66 and .85 for most modern coax, a half wavelength in coax could land between 11.88ft and 15.3ft. Not 18ft.

So does the velocity factor need to be calculated based on the parameters of a given coax? Or is it readily available on the cable itself? I've bought a 12' coax before( cut to make two 6' cables, was a total noob back then), but I've never seen a 15' one.

I guess the true resonant length depends on what channel you run too, eh?

 

[prcguy]

You can usually look up the velocity factor on the coax mfrs website. Most newer foam dielectric coax is in the 75 to 85% range and older coax with solid clear dielectric is usually around 66%. You shouldn't be worried about coax velocity factor or length unless your making a phasing harness or splitter for two identical 50 ohm antennas with 75 ohm coax.
prcguy

So does the velocity factor need to be calculated based on the parameters of a given coax? Or is it readily available on the cable itself? I've bought a 12' coax before( cut to make two 6' cables, was a total noob back then), but I've never seen a 15' one.

I guess the true resonant length depends on what channel you run too, eh?

 

[wa1nic]

Smith Charts

 

[SouthernRoller]

So basically for a single mobile ant, all that is needed is whatever length it takes to get from the radio to the antenna? Correct?
Also is it better to check SWR at the antenna or the back of the radio?

 

[JayMojave]

Hello Dump Trucker: Yeah what ever length that will reach, most antennas come with 18 feet to allow a long hook up I believe.

Its way easier to measure the SWR at the radio, as measuring it at the antenna would be a pain. Measuring the SWR at the radio end of the coax will show you just what the radio is seeing in SWR and is a valid ok way of measuring the SWR, and will indicate a valid ok condition or antenna coax problem.

Jay in the Great Mojave Desert

 

[wa1nic]

It is best to check the SWR at the antenna base, and not with an SWR meter but instead with an impedance analyzer.

If you have a non-reactive input impedance of say 40-60 ohms you will be good to go. You wont get that with a short antenna. You will only get that with a 7-9 foot antenna (9 foot being preferred). With a significantly shorted antenna, to get "50 ohms", you have to use a coil of some sort to add inductive reactance. Inductive reactance does not radiate.n "SWR METER" cant tell you if the antenna is reactive or not.

A better way to make a shorter antenna work is to still tun for resonance, giving you a lot lower than 50 ohm input impedance, but then use a matching transformer to step the impedance up to 50 ohms. You can make one with a ferrite torroid and some wire, or you can by a switch adjustable one from MFJ.

 

[prcguy]

Not a lot of folks have an impedance analyzer and especially CBrs. Yes its good to know the SWR at the base of the antenna as a sanity check but in most cases the antenna, the coax and its surroundings makes up the "antenna system" and I would go for the best SWR at the radio and not specifically at the antenna base.

If the SWR at the antenna is slightly different than that at the radio but you have the lower SWR at the radio and the SWR is stable when you move your hand up and down the coax, then go with the adjustment that gives a better SWR at the radio end. With mobile CB antennas its not very practical to measure the SWR right at the antenna anyway and I would only do that when investigating an antenna problem.

When a really short CB antenna will not match then applying tricks like a shunt matching coil at the base or using an impedance step down transformer as wa1nic suggested will help but again most CBrs don't have that or the expertise to fine tune a shunt coil. In the case of a short antenna that will not quite match or with an antenna with inadequate ground plane its much easier to fool it with a small value capacitor across the base of the antenna. Usually something in the range of 20 to 50pf does the trick and will allow you to get a good match.
prcguy

It is best to check the SWR at the antenna base, and not with an SWR meter but instead with an impedance analyzer.

If you have a non-reactive input impedance of say 40-60 ohms you will be good to go. You wont get that with a short antenna. You will only get that with a 7-9 foot antenna (9 foot being preferred). With a significantly shorted antenna, to get "50 ohms", you have to use a coil of some sort to add inductive reactance. Inductive reactance does not radiate.n "SWR METER" cant tell you if the antenna is reactive or not.

A better way to make a shorter antenna work is to still tun for resonance, giving you a lot lower than 50 ohm input impedance, but then use a matching transformer to step the impedance up to 50 ohms. You can make one with a ferrite torroid and some wire, or you can by a switch adjustable one from MFJ.