H. P. Friedrichs wrote:
I am interested in the origins and purpose of the "IRE (Institute of
Radio Engineers) Standard Dummy Antenna," as used during the alignment
of radio receivers.

I first came across this circuit in some old Hallicrafter's
documentation, which described the alignment of a particular radio. The
"IRE Standard Dummy Antenna" consists of four components--- a 200pf cap
in series with a 20 uH inductor, which links the signal generator to the
receiver's antenna input. A 400 pF cap in series with a 400 ohm resistor
is connected in parallel with (shunts)the inductor.

I gather that this circuit models a wire antenna of some type. Is this
true? What is the import of the component values and how were they
arrived at? I modeled the circuit in Spice and graphed the transfer and
impedance curves but it is not entirely clear to me what benefit there
is to using this circuit.

I've also Googled this and found some references here and there, but
nothing explains the "why" of it.

Is there anyone who can shed light on this?

73
Pete
AC7ZL

hpf (at) gainbroadband (dot) com

I built an I.R.E. dummy antenna recently to perform an alignment on a
Hallicrafters S-120. It is cobbled together on a small perfboard with
copper lands around the holes, has a length of coax with a male BNC
connector to go the the sig-gen and a female BNC connector to attach the
sig-gen lead.

I will post a pic on a.b.p.r when I can.

Bobby
KC9IHK

Pete, I was curious about the dummy antenna, too. Some time ago I
modeled it and checked the impedance looking back into the output leads
with a 50-ohm generator at the source. The results are he

http://users.tns.net/~bb/dummy.htm

Before I did the model I figured that the dummy antenna was trying to
simulate a certain length of wire on shortwave. But you can see that
the magnitude of the output impedance (upper plot) is around 400 ohms
throughout the shortwave part of the spectrum. Down in the broadcast
band the network becomes almost purely capacitive, which is what a
short wire will do.

In fact, at 500 and 1000 kHz (the two spots I checked), the dummy
impedance is almost exactly that of a wire that runs 15 feet vertically
and then 90 feet horizontally over perfectly conducting ground. I
imagine they had a 100-foot wire in mind, a figure I think I've seen
recommended for BC antenna length in the old days.

So I think the network is intended to be 400 ohms resistive at SW and
at BC behave as a 100-foot wire would. I align radios with the network
and then at installation I tweak the antenna capacitors, if they are
easily accessible, on the actual antenna.

Brian

Brian--

Your spice simulation looked familiar...VERY familiar. As it turns out,
I had plotted similar curves and happened to have used Microcap as well!
I was just not sure I understood the relationship between the transfer
function I saw and some kind of physical antenna.

Your comments make sense, though I do have one question: How did you
calculate the impedance of the antenna with the shape and dimensions
that you described?

73
Pete
AC7ZL


Brian wrote:

Pete, I was curious about the dummy antenna, too. Some time ago I
modeled it and checked the impedance looking back into the output leads
with a 50-ohm generator at the source. The results are he

http://users.tns.net/~bb/dummy.htm

Before I did the model I figured that the dummy antenna was trying to
simulate a certain length of wire on shortwave. But you can see that
the magnitude of the output impedance (upper plot) is around 400 ohms
throughout the shortwave part of the spectrum. Down in the broadcast
band the network becomes almost purely capacitive, which is what a
short wire will do.

In fact, at 500 and 1000 kHz (the two spots I checked), the dummy
impedance is almost exactly that of a wire that runs 15 feet vertically
and then 90 feet horizontally over perfectly conducting ground. I
imagine they had a 100-foot wire in mind, a figure I think I've seen
recommended for BC antenna length in the old days.

So I think the network is intended to be 400 ohms resistive at SW and
at BC behave as a 100-foot wire would. I align radios with the network
and then at installation I tweak the antenna capacitors, if they are
easily accessible, on the actual antenna.

Brian

Brian wrote:
The results are he

http://users.tns.net/~bb/dummy.htm

Nice charts, thanks for doing those.

So I think the network is intended to be 400 ohms resistive at
SW and at BC behave as a 100-foot wire would.

Pretty much what I said initially. A uniform high impedance.
As others pointed out the output attenuator of most "service"
grade signal generators at the time was all over the place.

What I find amazing is that what we can do with Spice in a few
minutes was painstakingly done by hand until the they got the
results they desired. Then published as a "cook book" standard
for others to use.

The bottom line was that you could introduce a signal into the
receiver under test without undue de-tuning of the tuned circuits.

Jeff


--
RESTRICTED AREA. Anyone intruding shall immediately become subject to
the jurisdiction of military law. Intruders will be subject to lethal
force, without warning, and on sight. USE OF DEADLY FORCE IS AUTHORIZED
under the Internal Security Act of 1950.

Your comments make sense, though I do have one question: How did you
calculate the impedance of the antenna with the shape and dimensions
that you described?


With a computer program.

Brian

Pete, I forgot to mention the reason I believe the IRE dummy antenna
tries to look like a 400-ohm resistive load at shortwave instead of a
certain length of wire. You can see why he

http://users.tns.net/~bb/antlen.gif

This is an old RCA chart showing the behavior of an end-fed wire for
lengths to 125 feet. It indicates the many resonances and
antiresonances a short wire can exhibit. Since the antenna impedance
and the resulting performance vary so markedly with wire length, and
because it would be unrealistic to expect all listeners to use one
particular length, I think the dummy antenna is just intended to
roughly match the input impedance of a typical radio. I'm not sure
about consumer radios, but old communications receivers all seem to
have a specified input impedance of either 300 or 400 ohms.

On the broadcast band, any wire shorter than about 150 feet will look
capacitive. It won't exhibit the resonances you see in the chart at
shortwave. So even if the wire is shorter than the 100-foot length the
dummy antenna seems to model and exhibits a higher capacitive
reactance, it won't affect the radio's RF tracking that much. On the
broadcast band, most radios seem to use rather loose antenna coupling
to minimize mistracking. This allows them to accomodate antennas of
various length. There is an interesting discussion about antenna
coupling strategies in the Radiotron Designer's Handbook.

Brian

GREAT info, that's why I lurk here often.

Just one teensy question, though...is this polarized?,
e.g. the 200pf cap is the receiver probe, the inductor
plus shunt is the signal gen connection? or versa-vice?

Thanks,
Terry Bakowski

[see you at the Peoria Superfest]


H. P. Friedrichs wrote:

I first came across this circuit in some old Hallicrafter's
documentation, which described the alignment of a particular radio. The
"IRE Standard Dummy Antenna" consists of four components--- a 200pf cap
in series with a 20 uH inductor, which links the signal generator to the
receiver's antenna input. A 400 pF cap in series with a 400 ohm resistor
is connected in parallel with (shunts)the inductor.

Brian,

This link seems to be broken.

Pete
AC7ZL


Brian wrote:
Pete, I forgot to mention the reason I believe the IRE dummy antenna
tries to look like a 400-ohm resistive load at shortwave instead of a
certain length of wire. You can see why he

http://users.tns.net/~bb/antlen.gif

This is an old RCA chart showing the behavior of an end-fed wire for
lengths to 125 feet. It indicates the many resonances and
antiresonances a short wire can exhibit. Since the antenna impedance
and the resulting performance vary so markedly with wire length, and
because it would be unrealistic to expect all listeners to use one
particular length, I think the dummy antenna is just intended to
roughly match the input impedance of a typical radio. I'm not sure
about consumer radios, but old communications receivers all seem to
have a specified input impedance of either 300 or 400 ohms.

On the broadcast band, any wire shorter than about 150 feet will look
capacitive. It won't exhibit the resonances you see in the chart at
shortwave. So even if the wire is shorter than the 100-foot length the
dummy antenna seems to model and exhibits a higher capacitive
reactance, it won't affect the radio's RF tracking that much. On the
broadcast band, most radios seem to use rather loose antenna coupling
to minimize mistracking. This allows them to accomodate antennas of
various length. There is an interesting discussion about antenna
coupling strategies in the Radiotron Designer's Handbook.

Brian

This link seems to be broken.


Pete, I remove temporary items from my web site pretty quickly due to
space restrictions. I tried to e-mail the chart to you but got the
following reply:


----- The following addresses had permanent fatal errors -----

(reason: 591 your host [216.86.143.7] is blacklisted by
bl.spamcop.net. No mail will be accepted)


No more midnight spam for me! Anyway, the chart is back up for a couple
more days.

By the way, on the perfboard I used to build my dummy antenna I added a
0.1 uF coupling capacitor between the signal generator input and a
second output lead with an alligator clip. This provides a 50-ohm
DC-isolated signal for IF alignment. I usually align radio IFs by
sweeping a signal at the front end, often just blasting through the RF
and mixer stages at 455 kHz. But the alligator clip is handy when I
want to drive the mixer grid directly. You can drive the grid with the
dummy network, but the 50-ohm source impedance of the direct output
ensures that the parallel impedance at the grid doesn't affect the
sweep flatness.

Brian