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View Full Version : Calling all telephone men - need velocity factor for jumper wire



AA7EJ
09-20-2011, 08:17 AM
I need help itentifying FOR SURE insulation material used in standard 24 gauge frame jumper wire.( White / orange color of unknown manufature )
I have build a temporary / test antenna ( array ) , and need to correct the model determined dimensions for the velocity factor.

All I know it is not a teflon.

The velocity factor can be anywhere from .66 down to .4 ( PVC)!

I would like to get as close as possible.

I suppose I could build a test dipole and compare the calculated and the actual resonance using GDM.

Any other idea?

( I am stuck with the wire !)

Thanks for reading.

KD8DEY
09-20-2011, 08:44 AM
Velocity factor deals with coax/feed line not insulated wire.

your thinking about capacitive effect when dealing with insulated wire on a dipole.

use the standard 468/f(mhz) formula to initially cut the wire.

after putting it up check the SWR to see where it actually resonates.

use the 2 frequencies to come up with a corrections factor

say you cut the antenna for 3.5mhz
468/3.5=133.71 feet

and after testing it actually resonates at 3.7 mhz

3.7/3.5=1.057

so
133.71
x 1.057

you would need to increase overall length to

141.35 feet

or lets say you cut the antenna and it resonates at 3.3 mhz

3.3/3.5=.942

so
133.71
x .942
=126.069 feet

468/f(mhz) is not an exact figure it is a compromise to get you within the ballpark because of all the variables such as soil and surroundings


if you want to use twisted pair for feed line you will probably need to use an analyzer such as the MFJ-259B and do a direct measurement and a little math as listed in the manual. But that would probably be so lossy that even RG-174 would look good

WA9SVD
09-20-2011, 01:21 PM
I need help itentifying FOR SURE insulation material used in standard 24 gauge frame jumper wire.( White / orange color of unknown manufature )
I have build a temporary / test antenna ( array ) , and need to correct the model determined dimensions for the velocity factor.

All I know it is not a teflon.



The velocity factor can be anywhere from .66 down to .4 ( PVC)!

I would like to get as close as possible.

I suppose I could build a test dipole and compare the calculated and the actual resonance using GDM.

Any other idea?

( I am stuck with the wire !)

Thanks for reading.

Not knowing the EXACT manufacturer, insulation thickness, actual insulation material, and other factors , there's no way to give you an exact figure.
Your best bet is to determine the capacitive effect empirically. There are no short cuts without making assumptions than can (and usually do) turn out to be false.

AA7EJ
09-20-2011, 03:04 PM
Thanks for replies.
I guess I failed to be more specific.
With all do respect - I did not ask for a lecture on how to calculate dipole dimensions.
( I seems to be getting that lecture often )

I did say "as close as possible".
I did ask for help from a telephone men who work with this stuff daily and should not need
detailed description.

And of course it is entirely my fault I no longer have the original plastic real with the label describing the wire.

I am sure I can estimate the data I need from measuring the resonance and from known length.

Let me revise my post

Would someone , telephone man preferred, read the label from 24 gauge (2 wire / 4 wire ) jumper / cross connect wire reel and see what is the insulation material used .
I do not need any other data , any color , but no teflon please.

I just need some way to estimate the velocity factor.

I have been unsuccessful finding ( Google) a picture of the reel with label visible.

Maybe trip to Home depot is due next or better yet to shop who sells other telephone material.
.


Never mind - the solution is so obvious - strip the insulation.

K7JEM
09-20-2011, 03:26 PM
I have two rolls of that stuff here, but niether has a label. I am almost 100% sure that the insulation is PVC, that is very common for all wire types of this nature.

The velocity factor is only a concern when you are using the wire as a feedline. The way it is twisted will also influence the overall length, since the wire is longer than a single piece of non-twisted wire.

Joe

KR2C
09-20-2011, 04:20 PM
This is what the label on my cross connect spool says:

GENERAL CABLE S 7023716
4C 24AWG 1000 FT.
CROSS CONNECT WIRE
BL/W-W/BL+O/W-W/O
(UL) CROSS CONNECT WIRE
XCW-2P24-S

The insulation appears to be PVC but might have some sort of wax coating too.

K8MHZ
09-20-2011, 05:24 PM
This is what the label on my cross connect spool says:

GENERAL CABLE S 7023716
4C 24AWG 1000 FT.
CROSS CONNECT WIRE
BL/W-W/BL+O/W-W/O
(UL) CROSS CONNECT WIRE
XCW-2P24-S

The insulation appears to be PVC but might have some sort of wax coating too.

According to General Cable, the wire is Type "F".

From Allied Wire:

Type F Wire
Low Smoke, Zero Halogen Silicone – 600 V, 200C

M16878/7 Wire Description:

M16878/7 Wire is designed to be flame retardant and meet Navy regulations on smoke, toxicity, and acid gas. Applications include those requiring high flexibility, durability, and high temperature applications.

M16878/7 Wire Ratings and Approvals:

Mil Spec: M16878/7
Temperature Rating: 150 C
Voltage Rating: 600V

NM16878/7 Wire Construction:

Conductor: Soft annealed silver plated copper
Insulation: Extruded silicone


I don't see any UL listing in my 2009 UL White Book for cross connect wire.

It would be nice if, in addition to having the UL label, it would also have a number, like UL 444, just for example:

http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=0444.html

There should also be some information on either the cable sheath or the conductor insulation as to it's type.

AA7EJ
09-20-2011, 05:29 PM
The velocity factor is only a concern when you are using the wire as a feedline.

Joe

I am not sure this is correct.
I like to have a discussion on that.

Can someone confirm it , please?

Why would this be? The mechanics / physics of current flow and resulting EMF should be same no matter what the role of the wire is.

Since the common opinion / fact is that the insulation is PVC - this would make a geat effect on the antenna since the velocity factor can be a little as .4.

Add the additional waxy coding on the wire, so it slips easy thru other jumper wires, I may as well use the worst scenario of .4.

The bottom line – it is hard to strip this insulation. It makes the wire behave like a rattle snake and streches in the process.

Lesson learned - it is a wrong , cheap, choise of wire for this antenna experiment.

AA7EJ
09-20-2011, 05:51 PM
It looks like Type F is "silicone rubber" or at least silicone based.

This type F stuff if fire retardant, PVC is dynamite by comparison!

Came to think of it - this jumper wire could not be PVC insulated!
The telephone industry was and hope still is pretty carefull after the famous
NY city telephone exchange fire years back.

Still searching for the elusive velocity factor for type F .

K7JEM
09-20-2011, 05:51 PM
I am not sure this is correct.
I like to have a discussion on that.

Can someone confirm it , please?



The velocity factor is important in a coax cable or an open wire feeder because it represents how slow the RF signal will travel, compared to the speed of light. You probably already knew that. But when you make an antenna out of it, the velocity factor becomes that of air (or close to it), since that is now the dominant insulator. The PVC (or whatever) is no longer much in play, since air is now between the antenna wire and the ground, or whatever close objects are around.

In a feedline, or this case twisted pair, the VF is mostly a function of whatever is insulating those two close wires. In this case, it is a fraction of an mm of PVC. But if you untwist the wires, and space them an inch apart, you can readily see that air is now the biggest factor, not the PVC.

I hope this is clear.

Joe

AA7EJ
09-20-2011, 06:11 PM
Thanks Joe,
it is clear now.
Since the velocity factor for the element wire or more precisely an air surrounding the wire is for all practical reasons 1
I have nothing to adjust and can make it play as is.

Lesson learned.
Over and out.

AC0FP
09-20-2011, 07:08 PM
Thanks Joe,
it is clear now.
Since the velocity factor for the element wire or more precisely an air surrounding the wire is for all practical reasons 1
I have nothing to adjust and can make it play as is.

Lesson learned.
Over and out.That's the way I do it. The insulation type is trivial for antenna wire.

fp

N3OX
09-20-2011, 09:19 PM
But if you untwist the wires, and space them an inch apart, you can readily see that air is now the biggest factor, not the PVC.


This is correct, but there ARE some antenna designs where the slight difference caused by the insulation will detune the antenna quite a lot.

A good example is the Moxon rectangle. It's a good example because there's a program out there (AC6LA's Moxgen, based on formulas from some of W4RNL's modeling studies) that will very precisely calculate the required dimensions for a Moxon. I would say that I usually get minimum SWR and best pattern to within 0.5% or so of where Moxgen says it will happen when I use bare wire (the required end insulators probably pull this a bit)

Many builders report they have to retune when they use insulated wire... the difference between bare and insulated is only a few percent, but 3% at 14MHz is 420kHz, enough to knock the tuning entirely out of the band!

That said, if you want to know the VF of a wire in any use, the easiest thing to do is measure it by comparing bare with insulated in whatever application.

AA7EJ
09-20-2011, 11:13 PM
This is correct, but there ARE some antenna designs where the slight difference caused by the insulation will detune the antenna quite a lot.

A good example is the Moxon rectangle. It's a good example because there's a program out there (AC6LA's Moxgen, based on formulas from some of W4RNL's modeling studies) that will very precisely calculate the required dimensions for a Moxon. I would say that I usually get minimum SWR and best pattern to within 0.5% or so of where Moxgen says it will happen when I use bare wire (the required end insulators probably pull this a bit)

Many builders report they have to retune when they use insulated wire... the difference between bare and insulated is only a few percent, but 3% at 14MHz is 420kHz, enough to knock the tuning entirely out of the band!

That said, if you want to know the VF of a wire in any use, the easiest thing to do is measure it by comparing bare with insulated in whatever application.

Good points.
Apparently the Moxon shape has something to do with it.

From the get go, I was actually more concerned about those "few percent", but got sidetracked for wrong reason.
It is funny, sometime I take a position not to sweat small things and this time I just went berserk the opposite direction.
Since this modified HB9CV array is based on Extended Double Zepp (EDZ) , the elements impedance being high anyway and relatively (1/8 wavelength ) apart will probably not change the property much.

I am after the radiation pattern and some changes elsewhere are the small stuff.

Thanks all for participating, from rocky start the discussion did the job well.
73

N3OX
09-20-2011, 11:42 PM
Good points.
Apparently the Moxon shape has something to do with it.

I'm not so sure about that. I feel like the Moxon is one of the few antennas where the "cutting formulas" are basically exact ;D Some other wire beams have the same issue:

http://www.leoshoemaker.com/hexbeambyk4kio/broadhexspecs.html

There's a shortening effect on dipoles and so forth but people don't usually check the actual finished length against any specific prediction, because it's easier to just trim.

If you do a lot of detailed model to measurement comparisons I bet you'd routinely notice the "few percent" of the insulated wire on all antennas. The Moxon's fiddly tip gaps might exacerbate the problem somewhat but I wonder if that's really the reason. What other antenna are people always trying to duplicate exactly according to a model prediction that would plausibly use insulated wire? People build VHF/UHF yagis to the nearest mm, trying to duplicate a model or proven built design all the time but they'd never use insulated rod.

In a lot of cases it doesn't matter in a practical sense but I think it pretty much always matters in the sense that you could detect the effect if you pay close attention to your exact tweaking. Moxons are just a striking example because you can calculate the exact dimensions in a few seconds for whatever bare wire you have laying around with a piece of free software. With bare wire Moxgen is dead on. With insulated wire it isn't.

If you look at the differences in K4KIO's table above that's similar. Granted, the G3TXQ hexbeam is also a close coupled beam so maybe that's relevant, but I think it bears taking a close look if you're really trying to duplicate something exactly.

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