Part 15 AM radiating counterpoise observations

[JasonW]

"To a kid with a Field Strength Meter, every piece of wire is an antenna!"

I'm well past being a kid (chronologically, anyway), but tonight my curiosity got the better of me.

I have Talking House www.talkinghouse.com and TalkingSign www.talkingsign.com Part 15 AM transmitters, and I was wondering if their AC adapters' power cords might be wired so as to function as counterpoises for the transmitters' 3 meter wire antennas.

If so, I reasoned, the transmitters could be set up with the power cords hanging vertically, so that the RF radiation from the power cords would augment the radiation from their antennas (rather like a stubby vertical dipole).

I set the TalkingSign transmitter for its highest frequency (1690 kHz) and adjusted its matching network for maximum radiated power, as indicated by the built-in RF ammeter. While broadcasting carrier only, I moved my Micronta Field Strength Meter so that its sensor antenna was only ~10 mm from the transmitter's power cord. The meter read "1" out of the relative Field Strength range of 0 to 5.

To make sure the meter was detecting RF from the transmitter and not residual fields from the 60 Hz AC, I de-tuned the antenna matching network while watching the meter. The Field Strength dropped as the antenna was de-tuned away from resonance, so the meter was detecting RF. The results with the Talking House transmitter set to 1700 kHz were identical (I quickly selected 520 kHz and saw the meter reading drop until the motorized automatic antenna matching unit tuned itself to 520 kHz, after which the meter reading went back up to "1").

Since the power cord does serve as a counterpoise on these transmitters, using an extension cord should increase the counterpoise radiation. I wonder what a 140 foot long extension cord, close to 1/4 wavelength at 1700 kHz, would do if it were run up and over a tree and then down to an outdoor AC outlet? (I'm not advocating such a set-up, but it's an interesting thought experiment.) -- Jason

 

[rfry]

> I set the TalkingSign transmitter for its highest frequency
> (1690 kHz) and adjusted its matching network for maximum
> radiated power, as indicated by the built-in RF ammeter.
> While broadcasting carrier only, I moved my Micronta Field
> Strength Meter so that its sensor antenna was only ~10 mm
> from the transmitter's power cord. The meter read "1" out
> of the relative Field Strength range of 0 to 5. (etc)
_____________

If this is a typical Part 15 AM tx system with the 3-m antenna connected directly at the tx, then the shield on the DC power lead probably is connected to the tx chassis, and will become part of the antenna -- to the extent that it can find an r-f ground through the power supply. The r-f current able to flow on that shield will cause radiation.

(added later) Also wires near the 3-m antenna (when radiating), and whether or not connected to the tx can act as parasitic radiators, and show a measurable field very close to them.
//

 

[JasonW]

On these transmitters, the 3 meter wire antenna is connected either to the RCA/phono antenna jack center conductor (TalkingSign) or to an internal screw wire clamp (Talking House).

What made me wonder about the power cord counterpoise was that the TalkingSign's power cord clearly has no grounded shielding braid sheath--the two wires are clearly visible side-by-side. The Talking House power cord is one round cord, but it is far too narrow to contain the two wires -and- a shielding braid. -- Jason

> > I set the TalkingSign transmitter for its highest
> frequency
> > (1690 kHz) and adjusted its matching network for maximum
> > radiated power, as indicated by the built-in RF ammeter.
> > While broadcasting carrier only, I moved my Micronta Field
>
> > Strength Meter so that its sensor antenna was only ~10 mm
> > from the transmitter's power cord. The meter read "1" out
>
> > of the relative Field Strength range of 0 to 5. (etc)
> _____________
>
> If this is a typical Part 15 AM tx system with the 3-m
> antenna connected directly at the tx, then the shield on the
> DC power lead probably is connected to the tx chassis, and
> will become part of the antenna -- to the extent that it can
> find an r-f ground through the power supply. The r-f
> current able to flow on that shield will cause radiation.
> //
>

 

[rfry]

> What made me wonder about the power cord counterpoise was
> that the TalkingSign's power cord clearly has no grounded
> shielding braid sheath--the two wires are clearly visible
> side-by-side. The Talking House power cord is one round
> cord, but it is far too narrow to contain the two wires
> -and- a shielding braid. -- Jason
______________

It wouldn't have to be a shield. All it takes is to be the "return" conductor for the DC power supplied to the tx.

 

[JasonW]

> It wouldn't have to be a shield. All it takes is to be the
> "return" conductor for the DC power supplied to the tx.

I know. If they had been intentionally trying to limit power cord radiation, they could have used a sheath that wasn't electrically connected to the transmitter chassis. The fact that they used no sheath at all (when they could have) aroused my curiosity. -- Jason

 

[radio8z]

> > It wouldn't have to be a shield. All it takes is to be
> the
> > "return" conductor for the DC power supplied to the tx.
>
> I know. If they had been intentionally trying to limit
> power cord radiation, they could have used a sheath that
> wasn't electrically connected to the transmitter chassis.
> The fact that they used no sheath at all (when they could
> have) aroused my curiosity. -- Jason
>
Hi,

Thanks for sharing your experiment. This affirms what rfry was saying.

For your consideration:

A "sheath" or shield will not keep the RF inside the cable unless the current on the inside conductor is perfectly balanced and opposite the current in the shield. This is not the case when the shield is connected to the transmitter ground and earth ground and it will radiate.

At VHF and above, the RF can easily be decoupled from the feedline shield by using a 1/4 wave choke sleeve, and is done to prevent feedline radiation so that the antenna pattern is not distorted, but this isn't practical at MW frequencies.

Neil

 

[JasonW]

That's what I meant. A "floating sheath" not connected to anything at either end would tend to attenuate radiation from the chassis-connected DC return wire inside. -- Jason

> > > It wouldn't have to be a shield. All it takes is to be
> > the
> > > "return" conductor for the DC power supplied to the tx.
> >
> > I know. If they had been intentionally trying to limit
> > power cord radiation, they could have used a sheath that
> > wasn't electrically connected to the transmitter chassis.
>
> > The fact that they used no sheath at all (when they could
> > have) aroused my curiosity. -- Jason
> >
> Hi,
>
> Thanks for sharing your experiment. This affirms what rfry
> was saying.
>
> For your consideration:
>
> A "sheath" or shield will not keep the RF inside the cable
> unless the current on the inside conductor is perfectly
> balanced and opposite the current in the shield. This is
> not the case when the shield is connected to the transmitter
> ground and earth ground and it will radiate.
>
> At VHF and above, the RF can easily be decoupled from the
> feedline shield by using a 1/4 wave choke sleeve, and is
> done to prevent feedline radiation so that the antenna
> pattern is not distorted, but this isn't practical at MW
> frequencies.
>
> Neil
>

 

[radio8z]

> That's what I meant. A "floating sheath" not connected to
> anything at either end would tend to attenuate radiation
> from the chassis-connected DC return wire inside. -- Jason


Hi Jason,

I predict the sheath as you propose won't attenuate radiation. Maybe you can do an experiment to prove me wrong.

Neil

 

[JasonW]

I already did, a long time ago. I have an old unused RG-6QS cable TV coax going up from my house to the utility pole across the street, and it is visibly disconnected from the CATV box up on the pole (it's clamped to the box and the last foot or so is hanging down). It makes a good Short Wave receiving aerial.

I once tried using this cable as an AM transmitting antenna with a Talking House. I tried it two ways, with only the center conductor connected to the transmitter and with the sheaths (RG-6QS has two concentric braids) and center conductor shorted together and connected to the antenna terminal.

Using only the center conductor as an antenna (leaving the sheaths "floating"), I got about one block less range at 1400 kHz - 1700 kHz than when I connected the sheaths and center conductor together. -- Jason

> > That's what I meant. A "floating sheath" not connected to
>
> > anything at either end would tend to attenuate radiation
> > from the chassis-connected DC return wire inside. --
> Jason
>
>
> Hi Jason,
>
> I predict the sheath as you propose won't attenuate
> radiation. Maybe you can do an experiment to prove me
> wrong.
>
> Neil
>

 

[radio8z]

> I already did, a long time ago. I have an old unused RG-6QS
> cable TV coax going up from my house to the utility pole
> across the street, and it is visibly disconnected from the
> CATV box up on the pole (it's clamped to the box and the
> last foot or so is hanging down). It makes a good Short
> Wave receiving aerial.
>
> I once tried using this cable as an AM transmitting antenna
> with a Talking House. I tried it two ways, with only the
> center conductor connected to the transmitter and with the
> sheaths (RG-6QS has two concentric braids) and center
> conductor shorted together and connected to the antenna
> terminal.
>
> Using only the center conductor as an antenna (leaving the
> sheaths "floating"), I got about one block less range at
> 1400 kHz - 1700 kHz than when I connected the sheaths and
> center conductor together. -- Jason
>
> > > That's what I meant. A "floating sheath" not connected
> to
> >
> > > anything at either end would tend to attenuate radiation
>
> > > from the chassis-connected DC return wire inside. --
> > Jason
> >
> >
> > Hi Jason,
> >
> > I predict the sheath as you propose won't attenuate
> > radiation. Maybe you can do an experiment to prove me
> > wrong.
> >
> > Neil
> >
>


Hi again,

I don't doubt your observation, but is it possible that the clamp to the pole box was grounding the sheath either due to the insulation being worn or through capacitive coupling creating an RF ground at that point?

Neil

 

[JasonW]

It could be. I'd have to look up there with my binoculars to see it closely enough to be sure. -- Jason

> > I already did, a long time ago. I have an old unused
> RG-6QS
> > cable TV coax going up from my house to the utility pole
> > across the street, and it is visibly disconnected from the
>
> > CATV box up on the pole (it's clamped to the box and the
> > last foot or so is hanging down). It makes a good Short
> > Wave receiving aerial.
> >
> > I once tried using this cable as an AM transmitting
> antenna
> > with a Talking House. I tried it two ways, with only the
> > center conductor connected to the transmitter and with the
>
> > sheaths (RG-6QS has two concentric braids) and center
> > conductor shorted together and connected to the antenna
> > terminal.
> >
> > Using only the center conductor as an antenna (leaving the
>
> > sheaths "floating"), I got about one block less range at
> > 1400 kHz - 1700 kHz than when I connected the sheaths and
> > center conductor together. -- Jason
> >
> > > > That's what I meant. A "floating sheath" not
> connected
> > to
> > >
> > > > anything at either end would tend to attenuate
> radiation
> >
> > > > from the chassis-connected DC return wire inside. --
>
> > > Jason
> > >
> > >
> > > Hi Jason,
> > >
> > > I predict the sheath as you propose won't attenuate
> > > radiation. Maybe you can do an experiment to prove me
> > > wrong.
> > >
> > > Neil
> > >
> >
>
>
> Hi again,
>
> I don't doubt your observation, but is it possible that the
> clamp to the pole box was grounding the sheath either due to
> the insulation being worn or through capacitive coupling
> creating an RF ground at that point?
>
> Neil
>