Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Wire Size  (Read 1357 times)
W4CPM
Member

Posts: 9


WWW

Ignore
« on: September 03, 2009, 10:05:32 PM »

I am getting ready to put up a 66 foot 40/ 20 meter wire dipole.  At first I was planing to use 14 gauge copper wire simply because that is what the salesman from r and l suggested it.  Then another ham recommended that I use 8 guage copper wire because hf radio waves travel on the outside of the wire and the increased surface area would increase gain.  Later I noticed that I have available to me a plenty long enough length of support cable like what is used for telephone poles (not sure of the guage).  How much better will the 8 guage be over the 14 guage? Will I get better performance by using the support cable instead of the 8 guage copper wire? Thanks and God bless.

W4CPM
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #1 on: September 03, 2009, 10:24:31 PM »

"Then another ham recommended that I use 8 guage copper wire because hf radio waves travel on the outside of the wire and the increased surface area would increase gain."

Stop listening to that guy.  That's an example of a statement that's technically true but totally useless.

A 14AWG wire dipole is about 98.6% efficient, within 0.06dB of perfect according to my EZNEC model.  8AWG does indeed reduce the loss (in other words, increases the gain)  The 8AWG dipole is about 99.3% efficient, within 0.03dB of perfect.

Does that extra 0.03dB gain you anything?  Not a darn thing, practically speaking.

In the case of HF dipoles, which have rather low current in absolute terms, it just really doesn't matter what size wire you use.  If you go to 30 gauge wire, it becomes 91.2% efficient and you are only 0.40dB down from perfect.  That's still essentially impossible to measure.  Any smaller than 30 gauge and you'll never keep it in the air (I used to use a 30ga doublet in an apartment).

There are a lot of "technical tidbits" that people seem to latch onto and supply out of context... of course, for ANY frequency, bigger wire has less ohmic loss.  But what really matters is not what has "more" loss or "less" loss, but *how much loss there actually is*

In the case of a half wave dipole, you'd probably have to use 36 gauge nichrome wire to see any noticeable reduction of gain from a sane wire size and material.  

Wire choice for dipoles is really a mechanical consideration.  They have slightly more bandwidth the fatter the wire, but you really have to go over to a "cage" style to get a useful bandwidth increase, and a regular thin wire dipole will cover all of 40m or 20m easily.

73
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB5JEO
Member

Posts: 805




Ignore
« Reply #2 on: September 04, 2009, 05:14:34 AM »

Mechanically (which, as stated above, is all that really matters), it's really a choice based on how long you want it to last and how much you want to pay. Not that the cost of any dipole is all that much, but wire specifically for longer wire antennas will resist stretching, resist breakage, and won't rust. Things like copper-clad steel is tough and cheap but something of a tangle to work with until you get it up, if you're not careful about handling it. Flexible woven antenna wire is perfectly strong enough, and very easy to work with, if more expensive. But plenty of common copper insulated wire, mostly 12, is available from most any metal recycler for the price of it copper resale price by weight, and it will last a long time.
Logged
AA4PB
Member

Posts: 12993




Ignore
« Reply #3 on: September 04, 2009, 05:24:47 AM »

The possible down-side to ordinary copper wire from a hardware store is that it is soft and over time it will stretch. You **might** find that your resonant frequency lowers a little after a few weeks in the air. You can pre-stretch it before cutting to length.
Logged
G3TXQ
Member

Posts: 1531




Ignore
« Reply #4 on: September 04, 2009, 07:05:44 AM »

Copper losses in a half-wave dipole increase as the frequency is lowered: although the "skin depth" that you mention gets bigger (reducing the AC resistance per unit length by the square-root of the frequency) the increased length of the dipole means there is more wire and therefore more total resistance, and the latter effect dominates!

So a bad case might be an 80m half-wave dipole using 26 swg wire: that has a total resistance of 14 ohms. But remember that the current along the dipole is sinusoidal, so we have to divide by two to get the effective loss resistance referenced to the feedpoint i.e. 7 ohms. Assuming the radiation resistance is around 65 ohms for an 80m dipole at 50ft, the efficiency will be 65/(65+7)=90% equivalent to a change in signal strength of -0.5dB

Whichever way you look at it, like Dan says, wire losses in an HF half-wave dipole are negligible!

73,
Steve G3TXQ
Logged
K3WACKY
Member

Posts: 59




Ignore
« Reply #5 on: September 04, 2009, 09:06:47 AM »

It would be fun to make an 80m dipole out of 3 inch aluminum pipe and see how that works. Expensive and not needed, but I would like to see the bandwidth.

I use steel copper clad wire 14 gauge for my dipole which also has a black insulation coating. Very hard to see and works great and protects the wire from the elements. It's been up for about 5 years, no stretching that I can see. Any exposed soldered connections to the budwig center were coated with the spray on plasti-dip, same color. Simple cheap antenna.

73!
Logged
K3WACKY
Member

Posts: 59




Ignore
« Reply #6 on: September 04, 2009, 09:25:48 AM »

correction, the wire gauge was 18 gauge
Logged
WX7G
Member

Posts: 6219




Ignore
« Reply #7 on: September 04, 2009, 10:19:02 AM »

K3WACKY:

Using and EZNEC model of an 80 meter dipole 50' above 'average' ground we find the 3:1 VSWR bandwidth to be:

#18 wire, 250 kHz
3" tubing, 425 kHz

Gain difference? EZNEC reports 0.01 dB but in reality there should be no difference.
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #8 on: September 04, 2009, 11:22:57 AM »

If you want to cover all of 80m a cage dipole is gonna be a lot cheaper than 3" aluminum ;-)

'Course if you've got a 200 foot tower and giant rotator to put that aluminum dipole on top of, well, that's a different story ;-)
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K9IUQ
Member

Posts: 2056




Ignore
« Reply #9 on: September 04, 2009, 01:33:01 PM »

N3OX:"Stop listening to that guy. That's an example of a statement that's technically true but totally useless".

Boy, Dan got that right. I have always used common house wire bought at the local hardware store. Always used stranded and insulated. It will stretch a little the first year but I use open wire feedline and never have worried about it.

Always used 12ga or 14ga. I prefer 12ga as it is much stronger, good especially for 130ft dipoles

Now then about color. I always used black and believe black insulated wire has about 2db gain over most other colors except maybe pink. But that is another thread.  :>

Never use solid wire for antennas and you will only use copperweld once, horrible stuff to work with.

Stan K9IUQ
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #10 on: September 04, 2009, 02:07:05 PM »

"you will only use copperweld once, horrible stuff to work with. "

Yeah ... but that statement can be interpreted two ways:

1) You'll never use it again because you don't want to untangle another (#$&$#(& springy ball of wire...

or

2) You'll never use it again because the only thing that will bring the antenna down is if the supports come down ;-)

Copperweld has its place.  I used to use an endfed random wire that used a tree as one support and counterweights on either end and good hard-drawn wire would snap at the tree every year or so.

Copperweld fixed that.... but it was a real pain to deal with because it did indeed come in a flat coil with twist ties and got away from me as I tried to carefully unspool 200 feet of spring
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB9CRY
Member

Posts: 4283


WWW

Ignore
« Reply #11 on: September 04, 2009, 03:38:40 PM »

And a properly grounded tower actually continuously bleed off static charge and actually makes the tower less likely to be hit.
Logged
W5ER
Member

Posts: 74




Ignore
« Reply #12 on: September 07, 2009, 11:44:18 AM »

Appears that CRY got his topics messed up.
Logged
W4CPM
Member

Posts: 9


WWW

Ignore
« Reply #13 on: September 07, 2009, 01:48:21 PM »

So I can expect the clear insulated speaker cable (14 or 12 AWG, not sure) to perform and last as long as 14 AWG copper wire specifically sold for antennas? Thanks for all the advice. God bless.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!