Antenna Wire Questions

[David, K3TUE]

I suspect this may be an ill conceived question, but I am trying to get away with as low visibility an antenna as I can in my location to avoid HOA notice.

I would like to know what is the smallest gauge wire I need to handle the current presented to a longwire for use on 160/80/40 AM at the following wattages.  If you know how/where I can come up with this info and how to understand it that would be a bonus.

wattages:
100w:
200w:
400w:
600w:
800w:

Brown insulation seems to hide in the trees best, but I may rig up something to slink back into the woods to be drawn out as needed by nice thin (invisible) monofiliment, especially if the wire i need to use has to be particularly thick.

Is stranded as effective as solid antenna wire?

Is insulation worth more than sealing out the weather to slow corrosion?

[W1RKW]

Dave,
I don't have any HOA's here but I ended up putting a 75m dipole initially with white rope as a suspension line.  It wasn't more than a week when one of my neighbors commented on my super high clothesline in my backyard.  As soon as possible I immediately purchased some of that black dacron line and the dipole became invisible with the exception of the little white insulator which is hardly noticable at all.  The copper wire I used was the typical stranded antenna wire you see at hamfests which should handle a good amount of RF. The wire eventually oxidized in no time and made the entire antenna invisible.  I had Bob N9NEO over not long ago and he couldn't find my dipole until I  pointed out where the feed line was and was able to see the antenna.  Having a ham look for a dipole is a good thing. If he can't find it you should be good to go.   My other antenna is hidden in the woods.  Nobody knows it's there including my wife.  Go with dark colors and you should be golden even when the leaves drop you shouldn't have a problem.  And I think the wire guage for the typical copper wire should be more than adequate.  The only problem with this wire though is it is soft drawn copper and probably will expand and contract with temp changes. Other than that you should be OK.

[Bacon, WA3WDR]

I had good results from a coax-fed 1/2 wave dipole of #22 gauge solid magnet wire with about 80 watts carrier output on 40 meters, way back when.  I was on Long Island, and WA1ION Mark said that I came in like CHU.  That antenna wasn't much, and it was about 20 feet high.

[WA1GFZ]

I put up bare #8 last fall 160 m vee and the wire is quite brown now. I would only use insulation over stranded wire.  I wouldn't use smaller than #14 if you want to keep it up in the air.

[Vinnie/N2TAI]

I have had an 80 meter dipole made out of 20 gage magnet wire up for almost 10 years now. It has oxidized to an invisible brown now. Yes it gets broken by storms from time to time I just solder it back together not hard. It is only up 20 -25 ft and have worked all the way down to the carribean on 80 meters. I run a Drake TR4 or a Heathkit DX100 with no problems

[KB2WIG]

I've found the following website to be of interest.      cebik.com

If you can get past the hokey graphick, check this out

http://www.cebik.com/fdim/fdim2.html


For 'new' wire ( shiney), stretch it out and grab a can of flat black spray paint.  Randomly spray the conductor to give it a splotchy look. The idea is to visible break up the wire outline. This helps 'flage it....  gud luck    klc 

[LURD]

Hello All,
     For dipoles, I always use hard drawn welding wire. It is copper coated steel and will not stretch. The steel eventioaly rusts in the wire, and blends in very well.
     The beam that I have up, I painted the blue that the sky always seems to be. Works very well also, as I have had no niebors complain yet!
     Over the years, I have had my share of "bitching." But my big concern is getting sued because "Little Johnny" down the street got cancer!

Best Regards,

LURD
 

[K1JJ]

Here's the goods, guys.

I ran a model for a 75M dipole at 60' high using wire sizes from #8 to #50 showing db loss.

The results speak for themselves. The losses don't really start to get significant until you get above #36 wire.

The next step is to convert the output power into amps at 50 ohms to see if the current will melt the wire like a fuse. See my example below and results.


Dipole LOSS in db for a given wire size (75M dipole):

#8 wire....... -.05 db  99% efficent
#10............ -.07db
#12............ -.09db
#14............ -.1db 97% efficient
#16............ -.13db
#18............ -.17db
#20............ -.21db
#22............ -.26db
#24............ -.33db
#26............ -.42db
#28............. -.52db  88% effficent
#30............. -.65db
#32............. -.81db  66%
#34............. -.99db
#40............. -1.8db
#50.............  -4.23db   31%

For a 100W / current example:

P= I^2 *R

(1.41 * 1.41) * 50 ohms = 100W

So, under "chassis wire" ratings, the current in a 50 ohm center fed dipole at the center is about 1.5A for 100W. That means #28-30 wire will be OK since #28 will handle ~ one amp. It will probably be OK with even #32 wire because of the open air cooling. But mechanical breakage becomes the main factor here.

See for wire size/amps :
http://www.powerstream.com/Wire_Size.htm

Question: Look at the wiresize/amps chart above. Why is there such a huge disparity between chassis wiring and power transmission line? Is it 60 cycle skin effect? If that's the case, then at 3.8mhz, the wire may have to be derated tremendously, like maybe #20 wire for 100W.  Any comments?


73,

T

[WA1GFZ]

Tom Vu,
Now consider skin effect. The rf will stay concentrated near the surface I figure about .001 inch deep on 160 meters and 1/2 that on 75. I agree with your numbers if it were DC.

[K1JJ]

Frank,

Looks like we posted the skin effect comments at the same time.

So, based on 3.8mhz, what is the minimum wire size you would recommend for 1.5A of RF current for 100W?  The mass and integrated area of  .001" thick skin effect depth is not very much!

T

[Steve - WB3HUZ]

Then consider the efficiency of no antenna compared to one made of thin wire.

[WA1GFZ]

HUZ is right but I figure at 160 meters .15 inch has the same effective skin resistance as #26 at DC. .15 inch is around #8 or so. The core is just acting as a heat sink for the surface currents.

[K1JJ]

HUZ is right but I figure at 160 meters .15 inch has the same effective skin resistance as #26 at DC. .15 inch is around #8 or so. The core is just acting as a heat sink for the surface currents.

Wow, that's quite a statement. Are you on narcotics?   

So, to get the same effect as using #26 wire at DC requires using #8 at 1.8mhz?  How about at 28mhz?  I can see why aluminum/ thick Yagis work so much better up there when the input impedances get real low.

What I may model is a dipole at 100khz scaled up for height above ground and length to see the relative db loss change that skin effect has. That should be interesting.

T

[Steve - WB3HUZ]

Also remember, the 1.5A or whatever the number was for 50 Ohms and 100 watts occurs ONLY a the center of the dipole. The current decreases from there on out. By mid-way out on each leg of the dipole, current is something like one-quarter this value. So, one would need to calculate current and associated losses across the entire length of the wire, to get the real loss.


I think some of the antenna modeling programs include wire loss. Does yours Tom? I think I have one that does.

[K1JJ]

I think some of the antenna modeling programs include wire loss. Does yours Tom? I think I have one that does.

Yes, Steve, wire loss is what I posted above for the various wire sizes on 3.8 mhz with a dipole at 60' high. Used AO modeling program. It integrates the whole length of the wire for it's calculations allowing for the current taper-off as it gets towards the ends.  These calculations most likely allow for skin effect, I wud think.

BTW, going to a shushi resturant is a hour with Joe, KA1BWO. He's never had shushi before and wants to try it. Another potential convert....

----------
Dipole LOSS in db for a given wire size (75M dipole):

#8 wire....... -.05 db  99% efficent
#10............ -.07db
#12............ -.09db
#14............ -.1db 97% efficient
#16............ -.13db
#18............ -.17db
#20............ -.21db
#22............ -.26db
#24............ -.33db
#26............ -.42db
#28............. -.52db  88% effficent
#30............. -.65db
#32............. -.81db  66%
#34............. -.99db
#40............. -1.8db
#50.............  -4.23db   31%


Ya know, a backwards way to calculate heat and current for wire diameters would be to use the db loss and spead it along the wire. For example, #45 wire has about a 3db loss. For a 100W signal, 50W would be wasted as heat. Then there's the center heat sinking Frank mentioned. Probably not worth the effort to figure out. Stick with #10-#12 if possible and be done wid it.


T

[Steve - WB3HUZ]

Sushi is good for at least 100 watts.

Thanks for the numbers. Somewhere, I have numbers on wire loss at RF. I'll post when I dig them up.

[WA1GFZ]

Ah wire taper yes I stand corrected we are working our way to 377 ohms.
Still as HUZ said small is far better than nothing.
no narcotics or bait for me

[Steve - WB3HUZ]

I'll do some pickled egg plant. That should be good for a kW or more! The more garlic, the better.

[Bacon, WA3WDR]

I heard a strong carrier coming from a sushi antenna once.  WA5ABI was on it.   

[WA1GFZ]

now that is QRO HUZ!

[Steve - WB3HUZ]

Wire loss at RF info, as promised.

Please note, the loss at DC is given in  milliOhms per meter. The RF losses are give in Ohms per wavelength. So, the loss is actually increasing with frequency, even the the numbers are smaller (compare the loss of #36 wire at 3.5 and 7 MHz. For any equal length of wire, the loss at 7 MHz is greater than at 3.5 MHz - multiply the loss number for 7 MHz by 2, since the wavelength at 3.5 MHz is twice that at 7 MHz).

You can see the loss increases greatly at RF, as compared to DC. Converting the loss for #36 wire at 3.5MHz to milliOhms per meter (mOhms/m) yields: (195.37 Ohms/Lambda)/(85.714286 meter/Lambda) = 2.279 Ohms/m = 2279 mOhms/m. Compare that to the 1356.4 mOhms/m for DC.



------------------------------------------ Ohm / Lambda--------------------
B&S dia | Dc.     1.825    3.5    7.0   10.1   14.2   18.1   21.2   29.0
AWG mm  | mOhm/m    MHz    MHz    MHz    MHz    MHz    MHz    MHz    MHz
---------------------------------------------------------------------------
36 0.13 | 1356.4 329.59 195.37 114.92  89.43  71.08  60.62  54.77  44.84
34 0.16 | 841.0  223.82 135.88  82.17  64.82  52.16  44.84  40.72  33.64
32 0.20 | 536.5  158.03  98.24  60.95  48.64  39.54  34.21  31.19  25.94
30 0.25 | 337.1  112.16  71.47  45.44  36.65  30.05  26.14  23.90  19.99
28 0.32 | 212.2   81.15  52.94  34.38  27.97  23.09  20.17  18.48  15.52
26 0.40 | 133.5   59.67  39.76  26.28  21.52  17.85  15.64  14.36  12.10
24 0.51 |  83.9   44.56  30.23  20.26  16.67  13.89  12.20  11.21   9.47
22 0.64 |  52.8   33.72  23.20  15.71  12.98  10.84   9.54   8.78   7.43
20 0.81 |  33.2   25.82  17.96  12.26  10.16   8.51   7.50   6.91   5.85
18 1.02 |  20.9   19.89  13.96   9.59   7.96   6.68   5.89   5.43   4.61
16 1.29 |  13.1   15.44  10.90   7.52   6.26   5.26   4.64   4.28   3.64
14 1.63 |   8.3   12.04   8.54   5.92   4.93   4.15   3.66   3.38   2.87
12 2.05 |   5.2    9.42   6.71   4.66   3.89   3.27   2.89   2.67   2.27
10 2.59 |   3.3    7.40   5.28   3.68   3.07   2.59   2.29   2.11   1.80
 5 4.62 |   1.0    4.07   2.92   2.04   1.71   1.44   1.27   1.18   1.00
 1 7.35 |   0.4    2.54   1.83   1.28   1.07   0.90   0.80   0.74   0.63
 2 5.40 |   0.0    0.73   0.53   0.37   0.31   0.26   0.23   0.21   0.18
 100.00 |   0.0    0.18   0.13   0.09   0.08   0.07   0.06   0.05   0.05
 500.00 |   0.0    0.04   0.03   0.02   0.02   0.01   0.01   0.01   0.01
--------------------------------------------------------------------------

AWG round copper wire resistance.

[WA1GFZ]

I was playing with one of our emi antennas the other day. It is a 2 meter biconical dipole used 30 to 200 MHz. We were trying to find the strongest field 1 meter away.
The elements get wider about 1 foot near the ends. The strongest near field ends up being out near the end where the element is the largest diameter. This make me rethink the beach QTH antenna. Maybe a cage dipole without dropping the ends.
Shorter but fatter.........like my butt.

[K1JJ]

Frank,

For a 75M dipole it shud make no difference at all in far field signal strength whether you are using a single #14 wire  or an elaborate cage. Maybe 0.1db becuz of wire loss, if that. I don't believe the pattern could possibly change.

All you'd be doing is confirming your neighbors suspicions that you're a ham nutcase. In a residential neighborhood, you have so much crap you can put in the air - call it the "ant crap factor", if you will. That cage dipole will take up too much of your allotment, taking away from other bands....   Let the new Yagis take the heat instead. 

T

[WA1GFZ]

Tom,
True for 75 meters but when you try to play on 160 does the cage do a better job than a dipole with ends hanging down. The ant will be 150 to 160 feet flat top.
Cage does lower the Q. I found that a big deal when I had my 2 wire rhombic at my parents house.

[Steve - WB3HUZ]

I'm betting the difference would be imperceptible. If I were to put up multiple wires, I'd do a snake arrangement to make the antenna look electrically longer vice a cage arrangement. Also consider linear loading. Two relatively short segments on each leg of the dipole will make your 180 feet look like a full 240 feet. You could feed it with coax if you wanted. Joe, N2YR, has done just this with great success using a much shorter antenna.

Tom,
True for 75 meters but when you try to play on 160 does the cage do a better job than a dipole with ends hanging down. The ant will be 150 to 160 feet flat top.
Cage does lower the Q. I found that a big deal when I had my 2 wire rhombic at my parents house.