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Dipoles and Heat

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Frank MacDonell

Dipoles and Heat

I am using a center fed dipole in the attic for a K3. Does the antenna
generate any measurable amount of heat during TX? Thanks.

--
Frank KD8FIP
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David Woolley (E.L)

Re: Dipoles and Heat

Frank MacDonell wrote:
> I am using a center fed dipole in the attic for a K3. Does the antenna
> generate any measurable amount of heat during TX? Thanks.
>

Yes. Subject to using suitable measuring instruments.

If the antenna is a reasonable length, a lot more heat will be generated
in the building structure (conceivably more than 50% of the power) than
in the antenna wire. If the antenna is, in particular, electrically
very short, a lot of the power could go to directly heating it.

I will also depend on the size and construction of the wire.

Why? If you are considering the total thermal load, I think the
building structure dissipation will be the most important factor.

--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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Guy, K2AV

Re: Dipoles and Heat

In reply to this post by Frank MacDonell

Probably the same as as a 50 watt light bulb, but spread all over all the
material in the attic. One could not measure it with any ordinary heat
measuring devices. Quite a bit of the misdirected RF would be spread all
over places other than the attic by the house wiring.

73, Guy.

----- Original Message -----
From: "Frank MacDonell" <[hidden email]>
To: "Elecraft" <[hidden email]>
Sent: Monday, May 25, 2009 6:08 PM
Subject: [Elecraft] Dipoles and Heat

>I am using a center fed dipole in the attic for a K3. Does the antenna
> generate any measurable amount of heat during TX? Thanks.
>
> --
> Frank KD8FIP
> ______________________________________________________________
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Jim Brown-10

Re: Dipoles and Heat

On Tue, 26 May 2009 18:06:03 -0400, Guy Olinger, K2AV wrote:

>Probably the same as as a 50 watt light bulb,

This seems wildly pessimistic. I'd guess no more than a dB or so of
dissipation. A typical dipole near resonance looks like about 70
ohms, nearly all of which is a resistance that represents RADIATED
power, with the rest being wire resistance. So lets say that some of
that power causes current flow in nearby conductive objects. That
current will be small unless those objects are near resonance, and
whatever the current, it will cause more re-radiation than
dissipation, because those objects have radiation resistance too.

73,

Jim K9YC

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AC7AC

Re: Dipoles and Heat

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Guy, K2AV

Re: Dipoles and Heat

In reply to this post by Jim Brown-10

The loss will be from dielectric loss, especially with asphalt shingles
above. For RF loss purposes, not much difference between layered asphalt
shingles and an inch, half inch, of dirt. Plenty lossy. Lay a dipole on an
asphalt roof and explain the detuning and the widening of the SWR curve.

73, Guy

----- Original Message -----
From: "Jim Brown" <[hidden email]>
To: "Elecraft" <[hidden email]>
Sent: Tuesday, May 26, 2009 7:08 PM
Subject: Re: [Elecraft] Dipoles and Heat

> On Tue, 26 May 2009 18:06:03 -0400, Guy Olinger, K2AV wrote:
>
>>Probably the same as as a 50 watt light bulb,
>
> This seems wildly pessimistic. I'd guess no more than a dB or so of
> dissipation. A typical dipole near resonance looks like about 70
> ohms, nearly all of which is a resistance that represents RADIATED
> power, with the rest being wire resistance. So lets say that some of
> that power causes current flow in nearby conductive objects. That
> current will be small unless those objects are near resonance, and
> whatever the current, it will cause more re-radiation than
> dissipation, because those objects have radiation resistance too.
>
> 73,
>
> Jim K9YC
>
>
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n7ws

Re: Dipoles and Heat

In reply to this post by Frank MacDonell

I responded privately to the OP earlier in the day with the following. Since this thread has taken on new meaning. I'm posting it here:

Frank,

Maybe a little thought experiment would help.

Consider that a K3/100 (I assume that is your worry) puts out at best 100 W and with most modes and operating styles, the average is much much less.

If you were running a full-carrier mode like FM or RTTY, the duty cycle is 100% while transmitting, but assuming a 50-50 split between transmitting and listening, it's 50%. So in the worst case, let's say the average output is 50 W. On CW or SSB it will be even lower.

Now this is at the output connector. With almost any run of cable, there will be some loss, but to be conservative, assume that it is zero. So at the antenna feedpoint the average power is 50 W. Hopefully, most of this is radiated and escapes the confines of your attic. In fact, a dipole is very nearly 100% efficient, but assume for a moment that it isn't and 50% of the available power is converted to heat.

This means that 25 W is dissipated as heat, and the other 25 is radiated. Most of the heat will be in the neighborhood of the feedpoint because that is the area of higher current. However, it isn't a point loss but spread over some length of the wire.

But let's pretend that all of the heat is generated in a very small spot on the wire and contemplate this.

I like to sometimes turn these things into other questions, so I would ask myself, "Self, if I was trying to heat up that wire to solder the transmission line to the wire, what is the likelihood of doing the job with a 25-W soldering iron?

Pretty unlikely isn't it? And that is the worst case scenario. With a small amount of power distributed over any appreciable length of copper there simply isn't going to be any significant temperature rise.

End Quote.

Now to address David's remarks that follow:

A lot of this is conjecture at this point since the size and composition of the attic, the electrical length of the dipole and the frequency are all unknowns at this point.

However, using my "backwards" logic from above, consider actually trying to absorb the transmitter power in the building without it leaking to the outside. Having done measurements in anechoic chambers and free-space antenna ranges, I can tell you that it's difficult.

Or try to imagine purposely attempting to heat the attic with a 50 W heating element. If it's a cold climate, it's not going to happen and if it's like where I live where last week the air temperature was 102F and the solar insolation on the roof was 1 KW/m^2, what effect would another 50 W make?

Answer: none. This is a non-issue.

Wes Stewart N7WS

> Frank MacDonell wrote:
> > I am using a center fed dipole in the attic for a K3.
> Does the antenna
> > generate any measurable amount of heat during TX?
> Thanks.
> >
>
> Yes. Subject to using suitable measuring
> instruments.
>
> If the antenna is a reasonable length, a lot more heat will
> be generated
> in the building structure (conceivably more than 50% of the
> power) than
> in the antenna wire. If the antenna is, in
> particular, electrically
> very short, a lot of the power could go to directly heating
> it.
>
> I will also depend on the size and construction of the
> wire.
>
> Why? If you are considering the total thermal load, I
> think the
> building structure dissipation will be the most important
> factor.

David Woolley (E.L)

Re: Dipoles and Heat

[hidden email] wrote:

> However, using my "backwards" logic from above, consider actually
+ trying to absorb the transmitter power in the building without it
+ leaking to the outside. Having done measurements in anechoic chambers
+ and free-space antenna ranges, I can tell you that it's difficult.

This isn't about high attenuations. The reasoning I used here is that,
if building structure losses were negligible, loft antennas would be
standard for the higher bands, rather than considered a last resort.

>
> Or try to imagine purposely attempting to heat the attic with a 50 W
+ heating element. If it's a cold climate, it's not going to happen and if
+ it's like where I live where last week the air temperature was 102F and
+ the solar insolation on the roof was 1 KW/m^2, what effect would another
+ 50 W make?

I'd agree that the additional heat load would be rather small, and I
would be curious to know the reason for the question.

PS. Your email program has a couple of problems. Firstly it is not
setting an In-Reply-To header, which is breaking threading of the list
when viewed with threading mail programs on the web interfaces.
Secondly it is using a non-standard end of line sequence in its quoted
printable encoding. That will probably be handled by the error recovery
in most software. (MIME encoding the end of line also goes against the
design philosophy of MIME quoted printable, namely that the message
should almost work when viewed by a plain text only email program.)
>

--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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Paul Gates-3

Re: Dipoles and Heat

In reply to this post by AC7AC

On Tue, May 26, 2009 at 8:10 PM, Ron D'Eau Claire <[hidden email]> wrote:

> Assuming the heat question was from a safety standpoint, I'd be more
> concerned with corona discharge than heat.
>
> Many attic antennas have very limited attention paid to insulating the
> high-impedance points, assuming the dry wood rafters are adequate. There
> are
> been reports of fires caused by just such assumptions where the high
> impedance end of the wire was near dust and other combustible materials.
>
> Ron AC7AC

Ron I did not realize that heat was a factor in an attic dipole. I lived in
an Apt complex in Lansing, Hich and I ran a 119ft dipole in

> Lansing, Mich. Fed with coax. The antenna wire was insulated #14 wire.

Paul Gates

>
>
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--
Paul, KD3JF
Glen Burnie, MD
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AC7AC

Re: Dipoles and Heat

In reply to this post by David Woolley (E.L)

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