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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 1:23 pm 
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Location: Freehold,ny 12431
Do they sell these? I have been reading about realigning a radio and have come across references to a dummy antenna? What is it used for and can you make one easy or buy one?<P>------------------<BR>


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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 2:26 pm 
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Location: St. Louis Missouri USA
The ones I've seen are usually home brew that can consist of a resistor and/or capacitor shunted across the antenna terminal.<BR>I've never used either. I just keep the signal generator's rf lead as far a possible from the radio to align it for maximum sensitivity, unless it is totally mis-aligned.<BR>Patrick<P>------------------<BR><BR>


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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 2:55 pm 
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Location: St. Louis Missouri USA
I forgot to mention that when I used a "dummy" it always loads down my signal generator where it will change the frequency as much as 40Kc. It appears that the generator doesn't have a buffer to keep the frequency from changing under a load.<BR>Patrick<BR><P>------------------<BR><BR>


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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 4:09 pm 
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Hi livin',<P>Do you have a name?<P>There are many different dummy antennas. The idea is to simulate the impedance of a real antenna when connecting the generator. There was an RMA standard, but not all radios conformed to it. Alignment instructions for many radios will specify the circuit to be used as a dummy antenna.<P>The load should not change the frequency of the signal generator.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR><A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A> | | <A HREF="http://www.Synchrophase.info" TARGET=_blank>http://www.Synchrophase.info</A>


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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 4:30 pm 
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Thanks for the info. <BR>Chris<P>------------------<BR>


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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 6:37 pm 
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For almost all Crosley radios, the dummy antenna specified is a 400 ohm resistor in series for AM broadcast bands and a .00025 cap in series on SW bands. The generator ground goes right to the chassis (assuming a transformer set) and through a .01 cap on AA5 sets.<P>------------------<BR>


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 Post subject: dummy antenna
PostPosted: Apr Thu 08, 2004 9:23 pm 
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Location: Circleville, OH, USA
I have posted the circuit of a dummy long wire antenna in the Photo Section. This circuit was adopted in 1938 by the IRE. Photo shows makeup & method of connection. With this between the radio & generator, the radio thinks it's connected to a long wire antenna.<P>------------------<BR>


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 Post subject: dummy antenna
PostPosted: Apr Wed 14, 2004 3:51 pm 
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Ken,<BR>Thanks for the picture.<BR>chris<P>------------------<BR>


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 Post subject: Re: dummy antenna
PostPosted: Nov Wed 16, 2016 2:19 pm 
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Hi guys. I've been reading about the dummy antenna.
I new here and not sure how to access the photo.
I'd like to make one.
Can anyone help ?

Terry


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 Post subject: Re: dummy antenna
PostPosted: Nov Wed 16, 2016 2:58 pm 
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The dummy antenna generally referred to in alignment instructions is a network specified by the RMA (Radio Manufacturers Association), and used between the signal generator output and the receiver antenna input.

Image

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 Post subject: Re: dummy antenna
PostPosted: Nov Wed 16, 2016 3:13 pm 
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A very complete treatment can be found here:

http://www.ax84.com/static/rdh4/chapte37.pdf

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 Post subject: Re: dummy antenna
PostPosted: Nov Wed 16, 2016 5:48 pm 
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Hi. I saw this thread and had a quick question. On the two aa5 radios that I have done, I just put a .1 uF cap as recommended between the high side of the signal generator and an alligator clip wire which attaches to the radio. Is this other procedure a replacement for that? I am just learning so figured I would ask now rather than later.. :)

thanks,
Mike


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 Post subject: Re: dummy antenna
PostPosted: Nov Wed 16, 2016 7:19 pm 
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mrtoad wrote:
Is this other procedure a replacement for that?

Some alignment instructions specify a coupling capacitor, some specify the dummy antenna, some don't specify either.

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 Post subject: Re: dummy antenna
PostPosted: Nov Thu 17, 2016 12:44 am 
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The alignment procedure for the radio i'm working on now, a Zenith 7-S-363 console, has a column in the procedure chart that lists the "Dummy Antenna" for that step.
i'm asking along with Mike, does that mean i should make an RMA dummy antenna for each of the capacitance listed for each step?
I found online what's called an IRE (Institute of Radio Engineers) standard dummy antenna.
I've attached a pic of the schematic of the IRE dummy antenna.
Thanks again guys


Attachments:
dummy ant.jpg
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 Post subject: Re: dummy antenna
PostPosted: Nov Thu 17, 2016 1:14 am 
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Most of the time the alignment instructions will tell you what to use: it's normally either a resistor or a cap. There's nothing to make up ahead of time as it varies from set to set. I just grab a resistor or cap as needed and return them to the parts drawers when done. I also will align with the signal generator not even connected and producing a barely audible signal (also visual via AVC on the VTVM).

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 Post subject: Re: dummy antenna
PostPosted: Nov Thu 17, 2016 2:43 am 
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Funny, I just wrote a little article about these for out radio club.
"Reprint" below if you are interested:

...And do I need one? The answer to that question is "maybe." If you spend much time aligning AM broadcast band or shortwave receivers, it is a good idea to have one of these little devices to help insure that the "front end" alignment that you do on the bench stays as close as possible to the same when the actual antenna is eventually hooked up to the receiver for use. This does not affect the I.F. alignment, but can affect the front end alignment. The reason for this is that a typical antenna that will likely be used with the receiver will have characteristics of capacitance or inductance at some frequencies. Since this is coupled in some fashion to the resonant tuning circuit inside the receiver's front end, when the actual antenna is connected it can detune the front end enough to cause your careful alignment job to wander a bit. This can cause the stations to appear in different places on the dial and also reduce the peak sensitivity that was tuned up with the trimmer capacitors of the tuning capacitor during the alignment procedure. Using a proper "dummy" antenna during alignment can avoid these undesirable changes when the actual receiving antenna is connected.

Unless you are sending a signal through space to an antenna similar to that which will be used on the receiver, the careful alignment that you do may or may not exactly hold when the antenna is connected for listening. Of course many vintage receivers that we restore use an integral loop antenna which must be in place for the radio to resonate with its tuning capacitor and pick up a signal. If this is the case, the usual procedure is to connect a loop of several turns of insulated wire, a few inches in diameter, to the signal generator and place the loop perhaps a foot away from (and parallel to) the receiver's loop antenna. This works well for injecting test signals into the receiver for alignment purposes because the receiver is using its intended antenna system with its associated impedance characteristics, the level of the signal is easily maintained at a low enough level to avoid overloading the receiver, and the loose coupling between the signal generator and radio receiver avoids loading down the receiver's resonant input circuit with the generator's low output impedance—which can cause a very broad and low level frequency range acceptance of the receiver.

But what if your receiver, like many vintage units, has only a pair of screw terminals of an external antenna, and no loop to radiate a test signal into? If you have the specific alignment instructions for this type of receiver, they may specify coupling the signal directly into the terminals through a small capacitance or a resistor. If this is in the instructions for aligning your receiver, there is certainly nothing wrong with doing it that way. If you do not have these step by step alignment instructions, the I.R.E. dummy is especially recommended. In past times, these were available, new, for purchase but I am not sure if you will be able to find one for sale at all these days. Not to worry, it is easy and fun to build your own, and the cost is minimal.
(Note: Vintage FM receivers with 300 ohm antenna inputs often specify a 150 ohm non-inductive resistor, such as carbon composition or carbon film, on each of the antenna terminals—In fact, this setup is the I.R.E. Standard Dummy Antenna for FM receivers operating on the VHF band. For AM broadcast band and HF shortwave receivers, the I.R.E. Standard Dummy Antenna for less than VHF applications is quite different, and that is the focus of this article.)
The I.R.E. Standard Dummy Antenna under discussion is, as previously described, for preventing these latter type receivers from becoming loaded down by the signal generator and by causing the receiver to "see" the test signals coming through a typical antenna, and not only helps keep an alignment job "true" when an actual antenna is used, but was/is also used for standardized receiver testing for sensitivity and selectivity of stations.

The impedance characteristics of the Dummy are shown in the graph below (From Radio Engineers' Handbook, Terman, 1943, McGraw-Hill, page 974) shows that at the broadcast band frequencies, the impedance is primarily capacitive, and similar to a long wire antenna. The impedance gradually levels off at around 400 ohms resistive at the shortwave frequencies to help match the typical 300 to 400 ohm impedances of vintage shortwave receivers.
Image

Image
The schematic for the I.R.E. Standard Dummy Antenna as shown below is from The Radiotron Designer's Handbook, 4th Edition, 1952, page 1299.

According to the Radiotron Designer's Handbook: (1) The component values should be within 10% of their nominal values. ( 2) The interconnecting leads between the generator and dummy antenna must be shielded, and also short enough to impose negligible voltage drop. ( 3) Stray capacitance between dummy antenna components should be minimized. This is all easily accomplished by using common "good RF practices" in keeping component leads short and allowing a bit of space between components and their surrounding housing (if any). The unit I built for my own use is actually inside a small metal box that is grounded to the common lead of the dummy antenna—though I do not believe this is really necessary for alignment purposes. It should definitely be inside a shielded metal enclosure for use for accurate sensitivity or selectivity testing by I.R.E. methods and standards due to the very small signal levels involved that may bypass an unshielded dummy antenna by radiation.

For components (referring to the schematic above), small ceramic disc capacitors may be used. Carbon composition, carbon film, or metal film may be used for the resistor(s). and the coil may be a hand wound air core coil. In my build, I used a 220pF ceramic disc for the 200µµF ( a pF, or picofarad, is just the modern term for µµF). I used two of the same connected and soldered tightly together in parallel for the 400 µµF. These are already 10% high, not including their 10% tolerance, but I thought it was close enough. I used a carbon film 390Ω. The 20µH (microhenry) coil was wound around a thin walled piece of polystyrene tubing, exactly 1/2 inch in diameter. Using 30 gauge enameled "magnet wire" that I bought at Radio Shack, it took 47 turns tightly wound next to each other in a single layer 1/2 inch long. I used a dab of clear, 5 minute epoxy to fasten the coil form to the bottom of my metal enclosure, and the other components are attached to the coil form. I did not take pictures of the inside of my unit at the time it was built, but below are pictures of the complete unit. Again, the enclosure is probably not necessary for most purposes.
This is a pretty easy little project for a rainy afternoon, and I believe it is well worth having this accessory on hand—though it doesn't get used "every day." I hope you found this article interesting and useful!
My I.R.E. Standard Dummy Antenna:

Image

Image


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