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VHF Coax Length

Old 10-05-2014, 02:08 PM
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Default VHF Coax Length

Among the items I've removed from my current boat to be installed on my new one is a VHF radio. I had the antenna on a swivel mount on a T-Top that was close to the electronic box in the T-top. left the excess coax as it was, laid it in a hank,(not loops) and left it laying on top of the elec box out of the way.
I would rather not have all of that unneeded coax creating clutter on the new installation.

Is the coax length critical to SWR on a VHF radio like it is on a ham or CB rig. On those it has to be left to a specific length in order to adjust SWR. I'm just wondering if I can cut it to just the needed length without degrading radio performance.
Old 10-05-2014, 02:32 PM
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SWR isn't usually adjustable on a VHF. Shakespeare recommends at least 3' of coax ...
Old 10-05-2014, 03:54 PM
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Thanks.. I figure I'll probably have 5-6 feet if I cut it to just what I need.
Old 10-06-2014, 11:41 AM
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Originally Posted by Bob Landry View Post
Is the coax length critical to SWR on a VHF radio like it is on a ham or CB rig. On those it has to be left to a specific length in order to adjust SWR. I'm just wondering if I can cut it to just the needed length without degrading radio performance.
Coax length has nothing to do with SWR - unless your impedance match is off. If your impedance match is off and you use a "tuner" to "correct" for that and the tuner has a hard time with the match then, yes, coax length can make a difference because the coax then is part of the impedance matching network. In a matched circuit (resonant antenna) coax length and swr has no application. You will always deal with loss in the coax - resonant or not - but at such a short length the loss is quite negligible.
Old 10-06-2014, 10:26 PM
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Transmission line length does not affect the VSWR on the transmission line. Your transmission line does not tune the antenna.
Old 10-07-2014, 02:40 PM
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Excess coax is generally coiled, rather than "hanked", so as to act as a common-mode choke, however minimal. Just don't coil in too small a radius, which can cause the coax to deteriorate, mostly from "cold flow" of the center conductor toward the shield/braid through the dielectric, which causes an impedance "bump".

I just acquired a Digital brand antenna (surplus, new-old-stock, almost free) intended for a BIG boat with 100' of coax pre-installed. That will definitely be shortened, rather than coiled, to use on a 23' boat.
Old 10-07-2014, 07:41 PM
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Originally Posted by jhebert View Post
Transmission line length does not affect the VSWR on the transmission line.
Coax, in the case of the vast majority of radio systems, is 50 ohms, or very close. If everything in the circuit is all 50 ohms (input/output of the radio and antenna feed point) then the coax does not have an affect.

In most common impedance mismatches the feed point of the antenna is not 50 ohms. In that case, and to the OP's original question, the coax does make a bit of a difference. Impedance is made up of two parts - a resistive component and a reactive component. The resistance is real = measurable, just like the resistance of the filament in an incandescent light bulb or a common thru-hole resistor. Reactance is imaginary. It is not measurable, directly as is resistance, because reactance only is "there" when the circuit is in use = there is RF on the line (AC). The current moving through the coax is sort of like electromagnetism - like the windings in a motor - magnetic fields collapse and build. In RF that cycle happens at the frequency of the signal, just like the AC outlet [60hz or there abouts] your computer is plugged in to (unless you're running on a battery, but how did you charge the battery?). When there is excess capacitance in the circuit the AC voltage leads the current. When there is excess inductance in the circuit the AC voltage lags the current. When the circuit is resonant and the impedance has no reactance you are left with pure resistance, the voltage and current are synced.

SWR is a measure of forward vs. reflected power. It doesn't measure impedance, but it is a fairly simple measurement comparatively which is why it is so commonly used. As an example - when you have a 2:1 SWR (most radio's "safe" zone, though under 1.5:1 is ideal) that means that the impedance can either be 1/2 of the nominal or 2x the nominal - so, for these radio circuits 50 ohms is nominal. That means a 2:1 SWR can be seen with either a 25 ohm load or a 100 ohm load = not matched.

Lets say your SWR is way off - like 11:1. Lets also say this is the high side, so 550 ohms. If you have a tuner that is rated for a 10:1 SWR or less it may struggle with that match. At such a high impedance you also have a very high voltage to go along with it. So what you can do is add coax (most common - a short jumper of a couple feet may be all you need). Going back to the reactance - whether it is capacitive or inductive, doesn't matter, you still have current and voltage that are separated. When you put a different length of coax in you shift the current and voltage and will probably find a spot where the tuner can operate happily. Manual tuners are notorious for this - you can arc capacitors with high impedance mismatches because of the voltages present. In shifting the match with coax you can keep the tuner from arcing. Very common practice.

I hope that sheds a bit of light on the subject of where the idea of changing coax length in an antenna circuit comes from.

In a manufactured antenna like all the marine antennas (fiberglass whips, smaller stainless whips, etc) they are all designed to be resonant. Without an SWR meter it would be hard to tell, but I would venture a guess that if the antennas are in good shape they will present very close to a 50 ohm match to the coax.

Originally Posted by jhebert View Post
Your transmission line does not tune the antenna.
Correct. I even hesitate using the term "tuner" above because it is a misnomer, but sort of like kleenex for tissue in the bathroom, even if it is kroger brand, everyone knows what it is. A "tuner" in my example is an impedance matching device made up of either a capacitor and inductor (L network) or 2 capacitors and an inductor (either a T network or a Pi network). They basically introduce false capacitance and inductance to cancel out those reactive components present in the transmission line going to the antenna. In doing so they fake the radio by making it "see" a "matched circuit" = it doesn't fold back power by seeing a high SWR. So, yes, you are correct that even at that point you are not "tuning the antenna". You're not - you are matching the RF source to the transmission line it is presented with.
Old 10-08-2014, 02:13 AM
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Sometimes the smoke and mirrors stuff can be a bit confusing.

If the antenna itself is resonant then it wont matter if you change the length of coax.

If it isn't resonant then changing the length of coax will indeed have an effect.

If you are want to change coax length and you don't know if the antenna itself is resonant or not then do it in 1/2 wavelength sections.

You do it that way because the same impedance is reflected every 1/2 wavelength.

For vhf marine, a free space half wavelength is 36 inches.

A 1/2 wavelength in coax is .66 of the free space measurement ( .66 being called the velocity factor ). Therefore a halfwavelength in coax is 24 inches.

Therefore cut it off in multiples of 24 inches (.66 x 36 inches) if are sure that you want to and re-terminate it with the connector.

If you cut off 1 x half wavelength (24 inches), 2 x halfwavelength (48inches) or 3 x halfwavelengths ( 72 inches ) you'll be able to accomplish what you want.

Don't take any other lengths off it and you'll be ok.

Take the cable off the end that plugs into the radio.

Last edited by mitch12; 10-08-2014 at 02:30 AM.
Old 10-08-2014, 05:12 AM
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Originally Posted by KC8QVO View Post
Coax...[very long reply]
VWSR is not changed by changing transmission line length.
Old 10-08-2014, 05:29 AM
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Originally Posted by jhebert View Post
VWSR is not changed by changing transmission line length.
My reply was to, hopefully, shed light on the OP's theory about the coax lengths and explain where it would apply, rather than the simple short answer that coax length does not affect SWR.

As per the OP's question:
Originally Posted by Bob Landry View Post
Is the coax length critical to SWR on a VHF radio like it is on a ham or CB rig. On those it has to be left to a specific length in order to adjust SWR.
I hope I didn't confuse or offend you , jhebert.
Old 10-08-2014, 07:31 AM
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Originally Posted by mitch12 View Post
Sometimes the smoke and mirrors stuff can be a bit confusing.

If the antenna itself is resonant then it wont matter if you change the length of coax.

If it isn't resonant then changing the length of coax will indeed have an effect.

If you are want to change coax length and you don't know if the antenna itself is resonant or not then do it in 1/2 wavelength sections.

You do it that way because the same impedance is reflected every 1/2 wavelength.

For vhf marine, a free space half wavelength is 36 inches.

A 1/2 wavelength in coax is .66 of the free space measurement ( .66 being called the velocity factor ). Therefore a halfwavelength in coax is 24 inches.

Therefore cut it off in multiples of 24 inches (.66 x 36 inches) if are sure that you want to and re-terminate it with the connector.

If you cut off 1 x half wavelength (24 inches), 2 x halfwavelength (48inches) or 3 x halfwavelengths ( 72 inches ) you'll be able to accomplish what you want.

Don't take any other lengths off it and you'll be ok.

Take the cable off the end that plugs into the radio.
That assumes the original coax is already a multiple of 24"...best to first measure from antenna end to verify. The goal is to end up with something a multiple of 24" after cutting.
Old 10-08-2014, 05:02 PM
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Originally Posted by KC8QVO View Post
I hope I didn't confuse or offend you...
I am not confused or offended.

Last edited by jhebert; 10-11-2014 at 09:08 AM.
Old 10-09-2014, 04:09 AM
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Question answered.

Last edited by Sea Ya Charlie; 05-29-2015 at 12:22 PM. Reason: Question answered
Old 10-09-2014, 11:34 AM
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Sea Ya Charlie

The collinear will not impact on the transmission line if it is resonant at the frequency you want to use it at. It'll be 50 ohms and you can connect any length of 50 ohm cable you like with cable loss the main consideration.

If the collinear is not resonant at the frequency you want then it will not be 50 ohms. You'll have a impedance mismatch and varying either the cable length or the antenna characteristics will affect the VSWR.

You can't change the resonance of the antenna by changing the feeder length.

You CAN adjust the cable length so that the combined load (coax and antenna) looks like 50 ohms. Then you'll be having the coax radiating as well as the antenna.

Hi Karl

It doesn't matter how long the supplied cable is to start with, if you take lengths of 24 inches (coax half-wavelength at VHF IMM frequencies) off it you'll still see the same VSWR as you did before you started.

Remember the rule that at electrical half-wavelength intervals on a piece of coax, the same impedance is reflected. In this case you are taking off multiples of a electrical half wavelength so nothing changes electrically except the cable gets shorter.

That applies whether the antenna itself is resonant or not.

regards

Last edited by mitch12; 10-09-2014 at 11:48 AM.
Old 10-09-2014, 12:27 PM
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I understand your point.

Also, don't assume a velocity factor of 0.66...many cables are quite different...I tend to use RG8X, which has a velocity factor of 0.84, for example. Some cables with same RG# have different velocity factors depending on whether foam or poly dielectric...RG8U can be either 0.66 or 0.78 for example. Can really throw-off calculations.
Old 10-09-2014, 11:13 PM
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Hi Karl

Probably so but the question here is simply RG58. Poor old Bob just wanted to know if he could cut off the surplus cable.

Answer; Yes .............if you go about it a certain way just to be on the safe side. Worst outcome is a new antenna..

Karl
I know you live on the outskirts of New York. Do you go to other places for holidays or fishing/boating excursions?

Last edited by mitch12; 10-10-2014 at 12:35 AM.
Old 10-10-2014, 12:04 PM
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Originally Posted by mitch12 View Post
...Poor old Bob just wanted to know if he could cut off the surplus cable. Answer; Yes...
And it won't change the VSWR, either.
Old 10-10-2014, 01:23 PM
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Originally Posted by mitch12 View Post
Karl
I know you live on the outskirts of New York. Do you go to other places for holidays or fishing/boating excursions?
My location is not what would be described as "the outskirts of New York", which I assume means NYC.

I actually own waterfront property on the "outskirts of Quebec"...I can swim to Vermont, and when I was in better shape, also swim to Quebec. For the record, I also own a house near Albany, still a several hour drive from NYC, that I maintain for employment reasons. My 2nd career has been in law enforcement and there isn't much crime that far north, except illegal border crossings, which the two agencies I work for have no jurisdiction in.

http://en.wikipedia.org/wiki/Rouses_Point,_New_York

I worked as an RF tech for two years before the career change, while finishing a master's degree in criminal justice. I have no technical credentials at all...everything self-taught. I've been a licensed ham since age 12, now Extra class, but am pretty inactive except for 20 meter CW. A tropical storm wiped-out all the trees that supported my wire antennas, though.

That's my story, and I'm stickin' to it....
Old 10-11-2014, 11:47 AM
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Found Rouses Point (not far from Quebec). Jeez Karl, you almost qualify to be one of our Canadian "Commonwealth Cousins".

New York State not New York City. About 300 miles from NYC. 150 miles from Quebec

In my state (Queensland), we'd class 150 miles as the outskirts but probably not 300 although its still not far.

Its a 1500 mile drive up the east coast of Qld and about 1000 miles across in the middle. Texas would fit into Qld twice. Alaska 1.5 times or so.

I went out to Longreach (centre of Qld) last week for an overnighter. In the last 60 miles (100 km) of road between Barcaldine and Longreach there were 5,800 kangaroo carcasses on the sides of the road in various stages of decomposition. Victims of collisions at night.

Carrion-eaters like hawks, crows and the occasional wedge-tail eagle all along the road. Hate to hit a late-take off wedgie unless square on the bulbar.

The road smelled like death.

Area is in drought and there's not a blade of green grass to be seen outside of the town. Been a long drought and area is dry. Getting progressively worse.

Ya gotta watch those tropical storms sweeping down from Canada.

ps Sorry Bob. Have you cut that cable off yet?
Old 10-11-2014, 02:19 PM
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Originally Posted by mitch12 View Post

I went out to Longreach (centre of Qld) last week for an overnighter. In the last 60 miles (100 km) of road between Barcaldine and Longreach there were 5,800 kangaroo carcasses on the sides of the road in various stages of decomposition. Victims of collisions at night.

Many people in the US believe 'roos are endangered and get livid when they hear of them being shot by the millions for dog food, and meat and hide export...few realize that some species are actually semi-endangered while the most common are grossly over-populated and a nuisance and danger. We have white-tailed deer problems in many areas, but certainly not to the degree you describe.

I broadsided one at 70 mph while riding a Honda VFR750...not a pleasant experience...I was covered with intestinal contents, the bike had to be towed, and I was 100 miles from home on a daytrip with no spare clothes...quite gross, but only minor injuries to me, but deer was cut in half. A local let me wash the shit off my clothes before the tow truck guy would even let me into the cab, and I had to sit on a plastic tarp. Ruined a $300 pair of leather pants, with $4K damage to a $7K bike. The bike stayed upright the whole time.

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