Do you really need 18 ft multiples of coax to tune your antenna?

[ModulationMaster]

I got 40 feet of LMR-400 will my SWR suffer?
because its not 54 feet (18 ft multiples) (Per-firestik)
It will be going to a solarcon a-99 on the roof

 

[binrat]

I just use whatever length is needed to make the connection. I have never bought into the 18' multiple thingy.

 

[medic18d]

I just use whatever length is needed to make the connection. I have never bought into the 18' multiple thingy.

ditto.

 

[vkrules]

http://www.worldwidedx.com/cb-antennas/163941-should-coax-cable-fine-cb.html You already asked this and got your answer.

 

[222DBFL]

It will work with dual antennas on a stormy night running a coba radio!! With the modulation master using it. Make sense now?? Stop trolling. How many people have to tell you it will work before you actually believe it yourself? Have you even tried using it yet??

 

[HomerBB]

Do you really need 18 ft multiples of coax to tune your antenna?

No.

Definition of NO

used to give a negative response.
"“Is anything wrong?” “No.”"
synonyms:absolutely not, most certainly not, of course not, under no circumstances, by no means, not at all, negative, never, not really;

 

[Lil'Yeshua]

I mentioned to a local that one only needs enough coax to reach what you're hooking it up to and not the 18' multiples. The guy flew hot at me saying that's a bunch of bunk and that he's been using the 18' multiple for 30 years. I left it at that.

 

[The DB]

If your not running anything more advanced than an SWR meter the length is irrelevant, irregardless of what Firestick says. If you are running something more advanced than an SWR meter then there is are lengths that can be used to reflect the proper impedance and reactance, however, 18 feet is not one of those lengths.

The problem with the calculations on the Firestick web site is they don't take into account two important factors. One of those is the velocity factor of the coax, which varies with different types of coax. Also, each frequency has its own set of lengths that work, and even neighboring channels need different lengths to be accurate. Yes they will be close, but their will be error factored in, and by the time you get half way through the band that error has added up fast.

All in all, be careful with the information that is on the Firestick web site, there is good information there, but there is just as much bad information, like that 18 foot length as a proper length...


The DB

 

[W5LZ]

"The guy flew hot at me saying that's a bunch of bunk and that he's been using the 18' multiple for 30 years."
Well, sounds like he 'defined' himself, doesn't it? Been doing it wrong for 30 years and proud of it!
Using any coax cable of some particular length is only beneficial in particular circumstances. Those circumstances deal with timing/phasing or resonance for some reason. All of those thingys are frequency specific, one size does NOT fit all. It may certainly work well on one channel, but it's benefit decreases as you move away from that channel.
Nothing new in the above, it's just said a little differently than before. It's been a fact for a very long time, but that 'magical' length sells coax! Imagine that...
- 'Doc

 

[Lil'Yeshua]

"The guy flew hot at me saying that's a bunch of bunk and that he's been using the 18' multiple for 30 years."
Well, sounds like he 'defined' himself, doesn't it? Been doing it wrong for 30 years and proud of it!
Using any coax cable of some particular length is only beneficial in particular circumstances. Those circumstances deal with timing/phasing or resonance for some reason. All of those thingys are frequency specific, one size does NOT fit all. It may certainly work well on one channel, but it's benefit decreases as you move away from that channel.
Nothing new in the above, it's just said a little differently than before. It's been a fact for a very long time, but that 'magical' length sells coax! Imagine that...
- 'Doc

Yep. Timing,phasing,and resonance would be the reasons why I would measure out a coax based on it's velocity factor for use say,like in a circular polarization antenna. I used to cut my coax long and then cut off the extra in increments in order to even up the swr readings on each end of the band I wanted to use it for but I think you or/and others commented on if that be the case then it would be because I had a non resonant antenna. All the more reason I would want to get an antenna analyzer.



P.S. If I do buy coax for specific application it would be cut based on velocity factor and not physical length

 

[M0GVZ]

Its because your coax was part of the antenna....

 

[Lil'Yeshua]

Its because your coax was part of the antenna....

Yes.

 

[HomerBB]

I usually use an electrical 1/2ƛ jumper for tuning antennas with the MFJ-259b analyzer.
When doing so, taking into account that the 1/2ƛ isn't determined by a length in general, but specific to the frequency of resonance I want the antenna optimized to, the jumper is not even close to 18' long.

I tune to 27.385.
Using this calculator - Wavelength Calculator K7TJO I get 215.5", or 17.96', as the frequency specific length.
Considering I use RG58 for the jumper, and the Beldin product I have is .66 VF (velocity factor) I then adjust with 215.5 x .66 = 142.23", or 11.8525'

This works out to nominal length of 11' 10.25"

The 18' general length is no where near the proper length for my purposes. When I want to tune an antenna, the e1/2ƛ jumper serves to permit me to read the condition of the antenna without transforming the the impedance through some other length, as would be the case with an actual 18'.
Even when wanting to use coax as a transformer for reasons pointed out by others in above posts, the same careful consideration for electrical lengths geared to the specific frequency, or near range of frequencies in use, must be considered and worked with. 18' just won't work as a broad generalization in virtually every case of use.

 

[Lil'Yeshua]

That's right HomerBB! Gee! I just remembered I used a Rat Shack two foot jumper to tune my A99.

http://www.bellscb.com/cb_radio_hobby/velfact.html

14.059153380978561 feet for my rg8X

 

[HomerBB]

Using a regular SWR meter there will not likely be a great variety of difference shown by the use of an off resonant jumper in your reading. However, when tuning for more than Standing Wave Ratio, ie, for the resonance of the antenna to a specific frequency utilizing an antenna analyzer, that length is critical.

What is often missed in the discussion of antenna coax length and SWR except by those schooled to the issue/discussion, is that SWR and resonance are not the same, and that SWR may not be an indication of the resonance of the antenna.

It is possible to achieve a low SWR and the antenna be resonant a significant distance along the band away from that low SWR point.
However, when the subject of coax length enters the discussion of antenna tune and performance, the matter of specific lengths must be understrood within the context of what is true or untrue about the influence of those lengths upon what is being read on an analyzer, or what is being transformed by those lengths for things like impedance adjustments, co-phasing, etc.