Mobile HF antenna melting....
 

Just installed a Hustler mobile antenna on my van and while tuning it,
it got so hot that the coil started to melt ( the cover) . What would
cause this? Antenna was mounted on a trailer hitch mount, 42" fold-over
mast, and with the 40 meter coil installed. At the time, the SWR was
less than 1.5 to 1, and running less than 100 watts. Is it normal for
these coils to run hot, even when matched?. The 20 meter coil started to
get hot too. If this is normal, how much power am I loosing in the coil.
Antenna seems to work well, everything else seems ok. Any Thoughts?
Pete N1AGV

On Sat, 02 Oct 2004 18:27:14 -0400, pete365 wrote:

Just installed a Hustler mobile antenna on my van and while tuning it,
it got so hot that the coil started to melt ( the cover) . What would
cause this?

Hi Pete,

A poor, resistive connection (resistance in general).

Antenna was mounted on a trailer hitch mount, 42" fold-over
mast, and with the 40 meter coil installed.

Which would lead to a radiation resistance (for a one fortieth
wavelength antenna) of 2 Ohms.

At the time, the SWR was
less than 1.5 to 1,

Which could be explained (by other than tuning) to have been achieved
by inserting a resistance of 30 or 40 Ohms.

and running less than 100 watts. Is it normal for
these coils to run hot,

Even 50W into 30 or 40 Ohms gets things toasty.

even when matched?.

A dummy load matches just as effectively and warmly.

The 20 meter coil

Which would lead to a radiation resistance (for a one twentieth
wavelength antenna) of 8 Ohms.

started to
get hot too.

And probably a lower SWR too.

If this is normal, how much power am I loosing in the coil.

Ah! The $64000 question. Probably all of it. The better question is
where is all the resistance coming from that makes this heat?

Antenna seems to work well, everything else seems ok. Any Thoughts?
Pete N1AGV

Antennas working pretty well and getting hot simultaneously should
give pause to re-examining the concept of "working pretty well" (by
what criteria, SWR?).

Follow the heat (literally). Wait for things to cool down and then
key-down for a shorter period and try to narrow in on that part that
warms first. Try one of those fish tank thermometers that is a sticky
press on type using liquid crystal phase shifting displays (AKA mood
ring technology). The heat is where the resistance is (and wire does
not often qualify except at a poor connection point).

73's
Richard Clark, KB7QHC

Just installed a Hustler mobile antenna on my van and while tuning it,
it got so hot that the coil started to melt ( the cover) . What would
cause this?

Any chance this is an old or defective antenna, with corrosion in the
winding?
Power dissipated in resistance rather than radiated, is what's making it get
hot.

--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR

Pete,
By any chance was the whip/upper portion of the antenna extended
down into the coil? As in a 'core' in the coil which shouldn't be
there, that sort of thing?
'Doc

I didn't think that the wip would go that far down. I guess what I'm
asking , is it normal for the coil to get hot when I go key down for a
couple of minutes. Has anyone had one of these apart? what kind of coil
is it? Pete N1AGV

'Doc wrote:


Pete,
By any chance was the whip/upper portion of the antenna extended
down into the coil? As in a 'core' in the coil which shouldn't be
there, that sort of thing?
'Doc

Richard Clark wrote:


Follow the heat (literally). Wait for things to cool down and then
key-down for a shorter period and try to narrow in on that part that
warms first. Try one of those fish tank thermometers that is a sticky
press on type using liquid crystal phase shifting displays (AKA mood
ring technology). The heat is where the resistance is (and wire does
not often qualify except at a poor connection point).


A very handy device for tracking this down would be an infrared
thermometer. Radio Shack sells one for between 30 and 50 dollars,
depending on whether it's on sale. It covers 0F to 400F, switchable to
C or F, and when held close to an object will measure the temperature of
a dime sized area or less.

I have never used mine for this purpose, but I have used it to check the
temperature of transistor pairs in RF amplifiers as well as heat sinks
in PCs. They are also handy to see if auto engine parts have cooled
enough to work on.

tom
K0TAR

This is strictly a guess, but I tried the Hustler on 75 meters when it first came out back in the 50s. I quickly
developed the impression that the principal impedance matching device was the lossy coil. VSWR was good, signal reports
were lousy. Needless to say, I went back to a bugcatcher and a base matching coil.

--
Crazy George
Remove N O and S P A M imbedded in return address
"pete365" wrote in message ...
I didn't think that the wip would go that far down. I guess what I'm
asking , is it normal for the coil to get hot when I go key down for a
couple of minutes. Has anyone had one of these apart? what kind of coil
is it? Pete N1AGV

'Doc wrote:


Pete,
By any chance was the whip/upper portion of the antenna extended
down into the coil? As in a 'core' in the coil which shouldn't be
there, that sort of thing?
'Doc

Thanks for the input , everyone. Pete N1AGV

Crazy George wrote:

This is strictly a guess, but I tried the Hustler on 75 meters when it first came out back in the 50s. I quickly
developed the impression that the principal impedance matching device was the lossy coil. VSWR was good, signal reports
were lousy. Needless to say, I went back to a bugcatcher and a base matching coil.

--
Crazy George
Remove N O and S P A M imbedded in return address
"pete365" wrote in message ...


I didn't think that the wip would go that far down. I guess what I'm
asking , is it normal for the coil to get hot when I go key down for a
couple of minutes. Has anyone had one of these apart? what kind of coil
is it? Pete N1AGV

'Doc wrote:



Pete,
By any chance was the whip/upper portion of the antenna extended
down into the coil? As in a 'core' in the coil which shouldn't be
there, that sort of thing?
'Doc