Runboard.com
Слава Україні!
Community logo


runboard.com       Sign up (learn about it) | Sign in (lost password?)

 
topbuzzuk Profile
Live feed
Blog
Friends
Miscellaneous info

Big Cheese

Registered: 11-2007
Location: Wiltshire, UK
Posts: 2655
Karma: 12 (+12/-0)
Reply | Quote
Wire antenna power ratings?


hello,,,, i was just wondering if anyone knew anything about what power you can shove down what wire? I used 12 gauge stranded wire and read that people used lots thinner. i was running a KL200 through my wire today and was wondering how i would know if it was too much for the wire, i dont think it is as the wire was bought for a car stereo/amp install.

---
26FB118
22/Apr/2008, 6:16 pm Link to this post Send Email to topbuzzuk   Send PM to topbuzzuk Yahoo
 
yamanx1 Profile
Live feed
Blog
Friends
Miscellaneous info

Head Honcho

Registered: 09-2007
Location: In a world of my own
Posts: 4894
Karma: 40 (+41/-1)
Reply | Quote
Re: Wire antenna power ratings?


Good question, I put 1.2kw through mine and its 12swg emoticon

---

By all means think yourself big, but never think others small.... R.F.Scott
22/Apr/2008, 6:28 pm Link to this post Send Email to yamanx1   Send PM to yamanx1
 
topbuzzuk Profile
Live feed
Blog
Friends
Miscellaneous info

Big Cheese

Registered: 11-2007
Location: Wiltshire, UK
Posts: 2655
Karma: 12 (+12/-0)
Reply | Quote
Re: Wire antenna power ratings?


crikey mine will be fine if yours hasnt popped like a fuse with that much power emoticon

---
26FB118
22/Apr/2008, 7:19 pm Link to this post Send Email to topbuzzuk   Send PM to topbuzzuk Yahoo
 
Hot Fusion Profile
Live feed
Blog
Friends
Miscellaneous info

Management

Registered: 03-2008
Location: South West England
Posts: 1389
Karma: 7 (+7/-0)
Reply | Quote
Re: Wire antenna power ratings?


Do an ohms law calculation.
50 ohms impedance, 5000 watts of power equals only 10 amps of current flowing at the feedpoint, and most wire commonly used for antennas can handle that without melting.

Also, that current is only present right at the centre, as there is an impedance gradient as you travel out along the wire, resulting in minimal current and maximum voltage at the ends. At these kinds of powers I suppose corona discharge may be a problem so good insulators would be needed at the ends.

But then - who runs 5KW??!

A more probable 800 watt power level gives rise to only 4 amps at the feedpoint.

So - don't worry about a wire antenna's power handling capacity. You will give up before it does! emoticon
22/Apr/2008, 8:24 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
yamanx1 Profile
Live feed
Blog
Friends
Miscellaneous info

Head Honcho

Registered: 09-2007
Location: In a world of my own
Posts: 4894
Karma: 40 (+41/-1)
Reply | Quote
Re: Wire antenna power ratings?


quote:

Hot Fusion wrote:

Do an ohms law calculation.
50 ohms impedance, 5000 watts of power equals only 10 amps of current flowing at the feedpoint, and most wire commonly used for antennas can handle that without melting.

Also, that current is only present right at the centre, as there is an impedance gradient as you travel out along the wire, resulting in minimal current and maximum voltage at the ends. At these kinds of powers I suppose corona discharge may be a problem so good insulators would be needed at the ends.

But then - who runs 5KW??!

A more probable 800 watt power level gives rise to only 4 amps at the feedpoint.

So - don't worry about a wire antenna's power handling capacity. You will give up before it does! emoticon



If this is so, and I dont doubt it for a minute, why do antennas, most antennas, give a power rating of 500 or 1000 watts? For instance a standard Silver rod is only rated at about 1000watts and its made from ali tube that would carry 1000's of amps?

Also voltage is an issue as well surely? You have quoted 4 amps at 800 watts, which is 200volts, so how do manufacturers arrive at their recommended power rating? I understand this if there is a loading coil maybe, but if the amp loading of the material used is the guiding factor, then most radio antennas will cope with just about anything you throw at it

 emoticon

---

By all means think yourself big, but never think others small.... R.F.Scott
22/Apr/2008, 8:44 pm Link to this post Send Email to yamanx1   Send PM to yamanx1
 
Hot Fusion Profile
Live feed
Blog
Friends
Miscellaneous info

Management

Registered: 03-2008
Location: South West England
Posts: 1389
Karma: 7 (+7/-0)
Reply | Quote
Re: Wire antenna power ratings?


Dunno, actually. emoticon

For end-fed antennas such as a cheap dipole, it must be the matching transformer/balun which is the weak point, bearing in mind recirculating currents which may be present, giving rise to far greater heating than the plain calculated current would suggest. I rewound the tank coil on one of my linears with 2mm diameter wire to increase efficiency, as the original coil used to get extremely hot due to these recirculating currents bouncing back and forth as they do. Even with the thicker wire, the coil still gets pretty warm.

For an antenna which is inherently a good match for 50 ohms such as an unloaded quarter wave vertical where there is nothing other than straight metal in the circuit, there is no real limit within the bounds of what is reasonable.

Or maybe it's a safety factor they are considering, and are thinking of the high voltages present? It'd be interesting to hear from a manufacturer on this - maybe they are obliged to place some limit on it, in much the same way that a tin of beans (which would remain edible for decades if unopened) has a definite expiry date...

Thoughts, anyone? emoticon
22/Apr/2008, 10:26 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
John193 Profile
Live feed
Blog
Friends
Miscellaneous info

Freebander

Registered: 11-2008
Location: South Glos
Posts: 260
Karma: 3 (+3/-0)
Reply | Quote
Re: Wire antenna power ratings?


This thread could open the proverbial worm can I think. As previously posted Tx, feed and antenna impedance is the key to overall current carrying ability. At 300 Ohms you get 6x the voltage than at 50, so current is 1/6. the losses are thus also reduced in the same proportion. Its why long commercial runs are 600, 1000, 3000 ohm hard parallel lines.
The downside is you are dealing in very very high voltages indeed, and lots of other safety issues start to rear their head.
I’ve tested a good number of antenna to destruction over the past 20yrs (scientific experimentation of course) and one of note was a GP27, it was rated at 1000wfm, ½ wave end fed.
At 480w the swr started to change, first it went down a bit! Then it rose alarmingly and the power fluctuated 50%-70% during TX for 20 seconds then went open circuit.
On recovery ten minutes after failure the matching coil was removed, the bobbin in it had melted and fused into a blob it was charred and smoking it was still very hot even after ten minutes. Further it’s connection to the ally had arced through the tube and burned a hole the size of an old ten pence piece. The base tube had curved over toward the hole. As you see it might not be the wire which is the weak link, it could be a former or a bobbin as in this case.
I’ve repeated this test a good number of times over a complete range of different manufacturers, similar results always pertain. Manufacturers are like MP’s they cant help themselves from lying through their teeth. 7.5db from a 5/8 ground image antenna! I ask you . We all know that its not physically possible to get more than 1.5db from a 5/8 in the physical world, so why lie? The answer can only be because too many folk don’t know, so its easy to b******t the public.
My advice would be to be extremely cautious of anything over 40% of rated or quoted figures. Wires are safer because there is no hidden coils, but remember the balun, 10 amps through a 4:1 balun could give 40 and that takes some carrying.
At 50 ohms assuming perfect radiation a 100w TX will pass only about 2 amps. With a tuner covering up a mismatch and an antenna feed resistance of say 20ohms it can climb very high indeed (5 amps) and you won’t notice it. Its not the fuse rating of the wire that’s as important as the feedpoint impedance.
If you halve the ratings you should be ok most of the time but with trany amplifiers on things get hot very quickly and often too quickly to react to.
Fb204 and I were ragchewing during a stormy night when I repeated an unscheduled test to destruction on a ½ w dipole. Smoke and pyrotechnics was the result of that one. 500w did that to a 1000w antenna!!! A sirio beam has a feed impedance after the gamma rod junction of 8-12 ohms that’s 125 amps at 1kw!!!
Ampacity of single 10awg=30A,12awg=40,14=25,16=18,18=10 all at 140degF max, awg is not quite swg their 10 is our 12, hope that helps.
John193.
 emoticon
27/Mar/2009, 1:00 pm Link to this post Send Email to John193   Send PM to John193 MSN
 
Hot Fusion Profile
Live feed
Blog
Friends
Miscellaneous info

Management

Registered: 03-2008
Location: South West England
Posts: 1389
Karma: 7 (+7/-0)
Reply | Quote
Re: Wire antenna power ratings?


A very well informed reply from John there, as we are beginning to expect. (Keep it up John, you are now officially our teacher!)

But as for the original question regarding the power rating of a wire antenna, the simple answer is to disregard it, as other components (balun, ATU etc) will give up way before the wire does.

Any wiser yet?! emoticon

---
Andy - 26 FB107
27/Mar/2009, 2:36 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 


Add a reply





You are not logged in (login)