A new ham on the HamRadioHelpGroup mailing list recently asked about what type of wire to use for building dipoles. There was a lot of talk about what the guy should use, including copperweld, FlexWeave (my favorite), and plain, old, 14-ga. insulated wire from one of the big-box hardware stores.
At that last recommendation, someone trotted out the old maxim that if you use insulated wire, you’ll have to make the antenna shorter, as the velocity factor of insulated wire is lower that the velocity factor of bare wire. This factoid has been part of ham radio lore for a long time. Is it true, though?
Well, not wanting to propagate (pun intended) an untruth, I decided to Google this. The second result was a posting by Reg, G4FGQ (SK), titled Velocity factor of an insulated wire” on radiobanter.com. Here’s what he posted:
First calculate P = Q / ( R + S )
where
Q = K * Ln( 4 * H / d )
R = K * Ln( 4 * H / D )
S = Ln( D / d )and
K = permittivity of insulating material.
H = height of wire above ground.
D = Diameter over insulating material.
d = wire diameter.All dimensions in same units.
Now P is the ratio of wire self-capacitance with insulation, to wire self-capacitance without insulation. It follows that –
Velocity Factor = sqrt( 1 / P ) along the insulated
wire.With ordinary antenna wire insulated up to twice the wire diameter with PVC, the reduction in velocity from the speed of light is insignificant and pruning can be forgotten about. It is much less than the pruning sometimes done for end effect which is usually unnecessary anyway.
To detect the effect precision measurements are necessary unless the insulation has a diameter of several inches or more. For the permittivity of the insulation to have full effect it would be necessary to completely fill the infinity of space with the insulating material.
So, it would appear that another old ham radio operator’s tale has been debunked. That other old hack, L(ft.) = 468/f(MHz) still rules. Have fun making dipoles.
Dan,
I am shocked. This means that not everything I read on the internet or hear on the ham bands is true. Now what I am going to do?
73, Bob K0NR
@K0NR: I know it’s a shock, Bob. You can always e-mail me, and I’ll set you straight. :)
Dan,
Dan, G4FGQ, passed away on Aug 29, 2006. K3HRN is maintaining his site information. Here is the URL: http://www.zerobeat.net/G4FGQ/ .
73,
Eric / K4PYR
I’m very sorry to hear that. Perhaps some note of that should be made on his QRZ.Com page.
I was always taught that if you left the ends open the RF could get out, but if you sealed the ends that it would not let the RF out. Also if you use insulated wire an inverted V lets the RF out better than a dipole. The RF flows out better at an angle. :) HIHI
Yes, it’s the opposite with components, if you let the smoke out they don’t work anymore.
Interesting point about the insulated wire. Always learn something new. This post about building things leads to a question I have. I am having my students (K5LBJ) build a vertical antenna modeled after the Minuteman 20. In getting parts, I am unable to find any plastic spacers to keep the coil evenly spaced. Is there such for sale? Are there some tips on how to keep a coil’s spacing while winding? Any help appreciated. We’re into new territory here, but the kids are having a blast.
73,
Ronny, KC5EES
Austin, TX
Hey guys, do not overlook the point! I just have put some numbers in this formula: K=2.5, h=2m, d=0,001m, D=0.002m and I got a P=1.052 and c=0.975. Its 2.5% at an altitude of 2m (imagine conterpois of antenna in a cind of portable use … there will be less height). So, tremendous difference at low heigts!!!
Yet more unadulterated garbage.. By the way NASA, the US military and renowned antenna expert L. B. Cebik, W4RNL all whole heatedly disagrees with your flawed assertion that the effects of wire insulation on antenna wire is insignificant ..
The effects of insulation on antenna wires is well known and to claim otherwise is a perfect example of the dumbing down of ham radio and explains why hams are no longer respected in the world of electronics, science or engineering.
By the way thanks .. After reading this blog a newly licensed ham I’ve been working with decided not to take my advice regarding reducing the conductor length on the 10m Moxon he was building.
His plan was to use some #16 AWG insulated THNN wire he had on hand. The program (MoxGen) he was using to calculate the dimensions only calculates dimensions for bare wire. Since he was using insulated THNN wire I suggested he reduce all dimensions by 3 percent..
This blog convinced him that I was wrong, as such he decided not to take my advice. The end results? His Moxon tuned up a whopping 630Khz below the calculated frequency for bare wire.
Unfortunately for him this was a hard lesson learned as he had already installed the antenna on a 50 foot push up mast on his roof.. This mistake cost him his day off, He had to pull the antenna down off the mast, disassemble the entire antenna and re-trim all the dimensions.
After reducing all the dimensions by 3 percent the antenna tuned up exactly on its designed frequency.
By the way using Cebik wire data EZNEC was able to accurately calculated the difference in resonant frequency between insulated and non insulated wire on his 10m Moxon model. EZNEC predicted that the frequency using insulated wire would come out 630Khz low and that’s exactly where it came out
Can you point to references that show that Cebik, NASA, and the U.S. military all wholeheartedly disagree? If so, I’d like to publish those here. Thanks, Dan.
I don’t know what you consider to be insignificant but the difference between insulated 12awg wire and bare copper wire when constructing a 20m Moxon is 1.4 Mhz. so if you are designing a Moxon and want it to be resonant at 14.200 and don’t take wire insulated into account then your antenna will come out closer to 13Mhz instead of 14,200. When modeling with say Eznec you should use a dielectric constant of about 5 and an insulation thickness of about .8 MM for THNN wire.
So clearly when building advanced wire antennas if you don’t take wire insulation in to consideration when calculating your antenna dimensions then your antennas won’t even come close to being resonant where you want them and in the case of the 20m Moxon it wont even be resonant in the ham bands . Now I don’t know about you but I would say that’s pretty significant
Oh and its not just morons, anyone who’s ever built a hexbeam and made the mistake of not taking wire insulation into consideration when calculating the wire lengths wound up with an antenna where none of the resonant frequency came anywhere close to the ham bands..
And its not just advanced wire antennas that are affected even a simple dipoles on 40m will see as much as a 200khz shift in calculated frequency between insulated and bare wire.. The dialectic effect of Insulation on wire being relegated to Folk lore and wives tails ? really since when has science and engineering been reduced to being called lore, This whole thread is a perfect example of the dumbing down of amateur radio ..
I’m not disagreeing with you, I’m just asking for references. If you’re supposed to take these things into consideration, how do you do so? If it’s a matter of science and engineering, there should be some equations somewhere. Rather than just calling people names, please provide some references.
Hi,
Yes, I’ve read the late Reg’s comments on VF. The reality is, every single antenna I have ever built with insulated wire ends up having to be shorter than calculated and usually less than 95% too. Even those designed with MMANA-GAL (which doesn’t let you specify a VF) end up shorter.
Now, whether this is a lower VF or end effects/environment is debatable, but shorter they are.
There is plenty of evidence that PVC-coated wire has a lower VF – EZNEC even allows for it. Its a shame that we can’t get that posting by Reg taken down as all the evidence points to it being plain wrong.
Steve G0KYA
Fromulas provided for Q,R,S and P are just plain wrong.
They indicate very thin insulation affects VF much more then very thick insulation.
This is opposite from physical reality.
Doesn’t the formula 468/freq account for the vf of the wire? Other wise it would be 492/freq …. i.e. it assumes a vf of 95%
I recently started to build a “shortened” dipole antenna based on a design that recommended #14 AWG, Stranded, Insulated wire. I thought this somewhat odd, because the same wire was being used for the inductors in the radiator. (I’d never seen inductors wound with stranded wire.) As I scratched my head, it also reminded me that I’d always wondered if there were any PROs or CONs regarding the used of Insulated Wire for antennas.
When I started to research “INSUL vs NON-INSUL”, I came across a short “One-Liner” in the ARRL Antenna Book (20th Edition), Page 4-31. It states, “Note that the use of plastic-insulated wire will typically lower the resonant frequency of a half-wave dipole about 3%.”
Although this statement is in the “End Effect” section, the particular paragraph that mentions the “3%” does not make it clear that “insulation” impacts “end-effect” and lowers the resonant frequency for a given physical length.
Using “Double Reverse Logic”, I almost convinced myself this would seem to say that the antenna would have to be constructed with a physical length of 3% longer, so that the lower resonant frequency would end up correct. The more I looked at it, the reverse seemed to win out.
The above said, I’m tempted to “start with” the longer hunk of wire prior to hauling out the Antenna Analyzer and starting to trim. (“I keep trimming this dipole and it’s still too short!)
?^)
73, Dave
Every wire antenna I’ve ever built with insulation since the 60’s when I got my first VSWR bridge comes up shorter than the 468 formula which only takes into consideration the length of bare wire in the real world plus the end effect shortening, The 492/f formula is in free space only.
I START at 3% shorter and then trim to a J0 resonance at whatever R results. Today I am using surplus submersible pump wire which is stranded #8 for the vertical and solid #12 for the radials; this is all free or at scrap prices from deep well installers in the area. It is also semi hard drawn copper so stretch wont be an issue as it is with soft drawn house wire.
Today I installed a full 1/4 wave sloping vertical for 160 hanging from a rope at the 160′ level of my 180′ tower. Cut for 1840 kHz (I enjoy CW DX and soon to try FT8) the 234 formula (half of 468) comes to 127.17′ which is close enough to an even 127′ at that low frequency. At higher frequencies fractions start to mean something.
With 4 resonant elevated radials at 20′ I wound up with 120.65 or a 5% shortening of the vertical and the radials being so close to ground (for 160) plus running over tree branches they came in at 112′, I tuned with two radials in a straight line fed as a dipole using an antenna analyzer and then the other pair at right angles.
Now that I have a reference I can install another vertical wire plus radials for 1950 kHz for some enjoyable AM QSO’s. I will also add a few more radials run sort of wandering to conform to property line and available trees; with 20 radials I should be fully efficient with negligible ground losses according to Rudi, N6LF’s extensive papers.
Carl
KM1H
Thanks for the tip about the scrap wire. With the price of copper these days, that could save some hams a lot of money.
Having always made my own antennas and a lot for vairing mil frequencies. These were always made using 7 strand 14# hard drawn copper. Once I became a civilian and did not have acces to hundreds of feet of the afore said copper wire , I had to start looking for wire at a reasnable price. This would normally be reels of mains cable with pvc coating. I remmembered having read up on the effect of coated copper wire making the antenna having to be cut shorter due to the vf of the plastic coating. Before getting back into Amateur radio, I had experimented with a 3 wavelength long delta loop for 11 meters. Having the available space at that QTH. This was first calculated by the recomended 1003/f (Ans x3). the antenna was more or les an equilateral triangle with approx 3ft of 6ins spaced centre tunning stub. The feed point chosen was at 1/3 up from the left side with a home brew 6:1 ballun into 50 Ohm coax. The tunning stub was the first thing to go. That was like 6ft shorter than predicted. I did get it right in the end with a 1:1 vswr. But I could have saved a lot of triming lengths of wire had I been able to know what the vf was going to shorten the total length. I suppose it is all good in the learning game of the Amateur Radio spirit of making antennas is about.
A little late to the game, but Rudy N6LF, who has gone through the software, and verified with hardware, says insulation increases electrical length by about 1.5%. I didn’t read the whole article, but it seems to do more with changing of the feedpoint impedance than the velocity factor.
http://rudys.typepad.com/files/insulated-wire-and-antennas.pdf
Dave AA3EE
Good ol’ Cebik has a very relevant article about this: http://on5au.be/content/amod/amod83.html
Seems like modeling theory suggests that the shortening effect is real, and that depending on WHICH model this effect can be large enough to matter from a practical standpoint.
Empirical observations (e.g. á la Carl
KM1H above) support the Cebik/NEC modeling results more than the G4FGQ modeling results. And empirical data are always the gold standard for testing theory.
Neill
AE7AE
Well… Never build a Moxon and put it at 50 ft. without “testing” it first! Cut long and trim, tried and trued!
You guys crack me up. Given all the possible variables, if you think that NEC, EZNEC or the likes thereof, or for that matter, fancy calculations will reveal an exact cut for an antenna, it will not. If it does, take the win! However, it will establish a good starting point or, depending on your expectations, may even be; “acceptable.” Everything will change once it starts to rain anyway so expect some variance.
Since magnetic energy is stored in the insulating material, it’s material composition and thickness are serious factors in design. What you must remember is that it’s much easier to shorten a wire than stretch it. That’s all I know and all I’ll say. 73’s all.