From: Will Martin <wmartin@stl-06sima.army.mil>
Subject: Re:  Info on AM antennas wanted -LONG-
Originator: wdp@gagme
Organization: US Army
Date: Fri, 20 Nov 1992 04:49:43 GMT


Here's some discussion we had a year ago over on rec.radio.shortwave
about the Select-A-Tenna and using it with the GE SuperRadio II.

Generally, the conclusion I've come to is that the Select-A-Tenna will
help (and in some cases *greatly* help) a marginal or mediocre AM radio,
especially one which is limited by a too-small ferrite-rod antenna built
in. But an already-good AM radio, like the GE SuperRadio II (note that
recent reviews lead one to think the new III is no better, and probably
worse, than the II) will not get much, if any, improvement with the
Select-A-Tenna. So if you are getting a radio like one of the new digital
shortwave models, which performs only moderately well on the AM BCB, the
cost of a Seletct-A-Tenna may be worth the investment to add improved MW
performance to it.

Regards, Will Martin

Following is a file of message traffic about this subject:

|Date:         Mon, 1 Jul 91 13:52:31 GMT
|Sender:       Short Wave Listener's List <SWL-L%OHSTVMA.bitnet@RICEVM1.RICE.EDU>
|From:         Will Martin <wmartin%STL-06SIMA.ARMY.MIL@RICEVM1.RICE.EDU>
|Subject:      Superradios, AM BCB DX< & the Select-A-Tenna
|
|All this discussion of the GE Superradio and other BCB DX topics inspired
|me to dig out my Superradio II and my Select-A-Tenna the other night.
|What ensued reminded me of why I had put the Select-A-Tenna away before...
|I'm hoping other people out there have this toy (or homebrew equivlents)
|and can tell me what I am doing wrong, or if the way it works is the way
|it is supposed to.
|
|For those who don't know what I'm talking about, the "Select-A-Tenna" is
|a tuned-coil AM BCB add-on antenna that inductively couples to a radio
|sitting next to it. It is supposed to enhance AM BCB reception. In form,
|it is a brown plastic disk, about a foot in diameter and two+ inches
|thick, which sits on little feet on its edge and which has a central
|tuning knob marked with an AM BCB dial scale. The instructions say to
|put it next to the radio, with its back facing the radio, and tune in
|a distant station on the radio , then tweak the Select-A-Tenna's tuning
|dial until you hear a sharp peak in the signal strength of the tuned
|station. You may have to tweak the radio and antenna a couple times to
|get everything optimized.
|
|Well, what I get is a sharp *null*, not a sharp peak. I seem to get a
|very low, dull peak next to the null, but it is far easier to tune in the
|null than it is to get the peak. In general terms, the peak isn't enough
|better than the eneral reception without the Select-A-Tenna to be worth
|the effort.
|
|Bad ASCII drawing follows:
|
|       ---------- Tuning across desired signal ----->
|
|                Dull peak
|
|                    _-_
|__________________/'   `\             ________________________
|                         \           /
|                          \         /
|                           \       /
|                            \     /
|                             \   /
|                              \ /
|
|                             Sharp
|                             Null
|
|(Null should be steeper and sharper than I can draw in ASCII; think of
|it as a sudden narrow cleft.)
|
|So it appears that this is doing just the opposite of what I need!
|The Select-A-Tenna is a sealed unit with no adjustments other than the
|tuning knob (the model I have is the one without a jack for an external
|longwire; there was such a variation, I know). Thus, I figure that I
|must be doing something wrong. I come up with a couple possibilities:
|
|1) The Select-A-Tenna works best with radios which do not have adequate
|antennas or front ends of their own. Thus it adds nothing to the good
|long ferrite rod and tuned circuits of the GE Superradio. (Evidence: I
|was also trying it with a Radio Sharck TRF AM radio with practically
|dead batteries. The Select-A-Tenna did seem to add more to the
|performance of that radio than it did to the Superradio.)
|
|2) I am arranging these units wrong. As I interpret the instructions,
|I tune the radio first to the station, and rotate it to maximize the
|signal (90 degrees away from the null [*]). [I was doing this on a non-metal
|TV tray the other night to make rotation easier.] I then sit the
|Select-A-Tenna at the end of the radio, a few inches away, with its back
|to the radio [that is, with the disc crosswise to the radio -- see below],
|and rotate the tuning knob to get the null/peak described above.
|
|Another Bad Drawing:
|(a view from above)                         ____
|                                            |  |
|                                            |  |
|           __________________________       |  |
|           |                        |       |  |
|           |     RADIO              |       |  ||
|           |                        |       |  ||
|           |________________________|       |  |
|                                            |  |
|                                            |  |
|                                            |  |
|                                            ----
|                                          Antenna
|
|Is this the right arrangement? Or should the Select-A-Tenna be parallel
|to the radio, at its back instead of at its side? Or should they be
|positioned as shown but the Select-A-Tenna be oriented toward the signal
|and the radio put in the "null" position?
|
|The instructions say to put the Select-A-Tenna "6 inches from the radio".
|What does that mean? 6 inches from the side? 6 inches from the end of
|the internal ferrite rod antenna in the radio? 6 inches from the *center*
|of that ferrite rod? Or where?
|
|I've tried moving the Select-A-Tenna around the radio to various
|positions but nothing seems any better.
|
|I'd appreciate comments and advice from other users of the
|Select-A-Tenna. It just doesn't seem to be working for me...
|
|[*] I can never remember; does the "null" happen when the ferrite rod end
|is pointing toward the station, or when it is crosswise to the station
tuned? 
|
|Regards, Will
|wmartin@stl-06sima.army.mil OR wmartin@st-louis-emh2.army.mil
|
|
|Date: Mon, 1 Jul 91 13:05:31 PDT
|From: ptsfa!dmturne@ns.PacBell.COM
|Subject: Re: Superradios, AM BCB DX< & the Select-A-Tenna
|Organization: Pacific * Bell, San Ramon, CA
|
|In article <9107011354.AA17135@cs.utexas.edu> you write:
|>Well, what I get is a sharp *null*, not a sharp peak. I seem to get a
|>very low, dull peak next to the null, but it is far easier to tune in the
|>null than it is to get the peak. In general terms, the peak isn't enough
|>better than the eneral reception without the Select-A-Tenna to be worth
|>the effort.
|
|Curious.
|
|>1) The Select-A-Tenna works best with radios which do not have adequate
|>antennas or front ends of their own. Thus it adds nothing to the good
|>long ferrite rod and tuned circuits of the GE Superradio. (Evidence: I
|>was also trying it with a Radio Sharck TRF AM radio with practically
|>dead batteries. The Select-A-Tenna did seem to add more to the
|>performance of that radio than it did to the Superradio.)
|
|Loop antenna performance is related to the effective size of the loop.
|The permeabiliby of the material within the loop is important to the
|effective size. Air has a permeability of one; some ferrites might be
|900-2000. An air core loop must have a much larger area than a ferrite
|one with equal performance. 
|
|>2) I am arranging these units wrong. As I interpret the instructions,
|>Is this the right arrangement? Or should the Select-A-Tenna be parallel
|>to the radio, at its back instead of at its side? Or should they be
|>positioned as shown but the Select-A-Tenna be oriented toward the signal
|>and the radio put in the "null" position? 
|
|The plane of the loop (Select-A-Tenna) must be parallel (and coaxial) with
|the plane of the turns of wire in the internal (ferrite) antenna.
|Before ferrite loop antennas became popular there was an internal loop
|antenna that usually was flat-wound and attached to the back of the radio.
|When radios used tubes and a large speaker this would allow the antenna
|to be at least 5 inches by 9 inches (or more). Since the internal antenna
|was flat and mounted on the back of the radio, the Select-A-Tenna should
|be placed at the rear of the radio.
|
|>The instructions say to put the Select-A-Tenna "6 inches from the radio".
|>What does that mean? 6 inches from the side? 6 inches from the end of
|>the internal ferrite rod antenna in the radio? 6 inches from the *center*
|>of that ferrite rod? Or where?
|
|The two antennae are inductively coupled; the closer the coupling the
|greater the loading on the Select-A-Tenna and probably the broader the peak.
|
|>[*] I can never remember; does the "null" happen when the ferrite rod
|>end is pointing toward the station, or when it is crosswise to the
|>station tuned?
|
|The "null" occurs when the plane of the coil winding is perpendicular to
|the transmitter. The signal is received equally at all points on each
|turn which causes the signal on one-half of a turn to be negated by the
|equal and opposite signal on the other half.
|---
|Dave Turner	415/823-2001	{att,bellcore,sun,ames,decwrl}!pacbell!dmturne
|
|
|Date: Mon,  1 Jul 91 16:22:09 EDT
|From: prmount@attmail.att.com (Paul R Mount)
|Subject: Re: Superradios, AM BCB DX< & the Select-A-Tenna
|
|I think you want the Select a Tenna next to the Superadio so that both
|are facing you.
|
|The Superadio AM antenna runs the length of the radio, right? (I have
|the original Superadio).  Now imagine the contents of the s-A-t
|as a horizontal bar inside it.  Position the two to effectively
|make one long ferrite rod antenna, with a six inch separation
|between the two halves.
|
|Looks like it was as you speculated you were nulling with the SaT what
|you peaked by rotating the GE.
|
|At leastyou have something normal to use.. I 've heard of people 
|who enhance their reception by playing around with a pair of scissors!
|
|Paul   (DXing from the other coast)
|
|
|Date: Mon, 1 Jul 91 14:24:57 PDT
|From: dmturne@ptsfa.PacBell.COM (Dave Turner)
|Subject: Re:  Superradios, AM BCB DX< & the Select-A-Tenna
|
|>When you refer to "the plane of the coil winding", you are meaning across
|>the ferrite rod, right? So the null is when the rod end is pointing toward
|>the transmitter, right?
|
|Right.
|
|>(I keep remembering flat ferrite bars wound with wire -- the "plane"
|>in that case could be interpreted as slices thru the bar but also as
|>the plane of the wires all lined up along the side of the bar, and 
|>thus be a plane parallel with the bar side.)
|
|To determine the plane of the loop, think of a single-turn of wire on
|the loop.
|
|The flat loops I'm referring to are wound such that the coil is one turn
|long and each turn is at a greater radius than the previous one (for
|circular loops). You might think of this as a flat spiral coil.
|A circular flat loop looks like an annular ring (if that helps).
|Most of these loops were not circular but oval.
|
|Ferrite antenna rods have either a round cross-section or a rectangular
|cross-section and are much longer than they are wide. The wire is always
|wrapped around the cross-section (the narrow dimension); the plane
|is parallel to any one turn and will be perpendicular to the long dimension.
|
|In any case, the turns of wire in the external antenna must be coaxial
|with the turns of wire in the internal antenna and close enough to be
|inductively coupled. Both the radio and the external antenna must be
|rotated together to achieve the best reception.
|
|Good luck.
|
|
|From: J G Miller <miller@ria.ccs.uwo.CA>
|Subject: Re: Superradios, AM BCB DX< & the Select-A-Tenna 
|Date: Tue, 02 Jul 91 17:58:03 -0300
|
|Intuitively I would have thought that you should 
|
|1) Tune into a weak AM station.
|2) Rotate radio until signal peaks.
|3) Position slect-tenna at back of radio and parallel to ferrite bar,
|   6 " from the ferrite bar.
|4) Adjust the tuning on the select-tenna until the signal peaks.
|
|Try this and please post your results.  I am interested in
|knowing if this device does improve reception over the internal
|ferrite rod antenna alone.
|
|Have you tried connecting up an external long wire antenna and
|ground to the terminals on the back of the radio for comparison?
|
|Greg Miller
|
|
|From: pyuxa!hwd@bellcore.bellcore.com
|Date: Thu, 4 Jul 91 09:42:40 -0400
|Subject: Re: Superradios, AM BCB DX< & the Select-A-Tenna
|
|That's kind of strange. I have the Select-A-Tenna and it
|performs rather well, giving a nice sharp peak.  The
|design, and in fact the antenna itself, have been around
|a long time.  It wasn't until recently that I purchased
|one of the antennas, but I remember them from many years ago.
|
|Harold
|
|
|From: HAVANAMOON%cup.portal.com@RELAY.CS.NET
|Subject: Re: Superradios, AM BCB DX< & the Select-A-Tenna
|Date: Thu,  4 Jul 91 09:56:37 PDT
|
|The "Select-A-Tenna" enhances(?) reception to some degree by
|enabling you to "null" out QRM from adjacent (or "on-channel") stations.
|
|I've never had much luck with devices such as this.
|
|73s
|HM
|
|
|From: ihlpm!rats@att.att.com
|Date: Tue, 3 Sep 91 12:12 CDT
|To: att!STL-06SIMA.ARMY.MIL!wmartin
|Subject: Re: Select-antenna
|
|I think it was you who asked a couple of weeks back about the proper way
|of connecting a SELECT-ANTENNA inductively to a receiver with a loopstick
|antenna. I happen to come across an article in a UK magazine, Radio & 
|Electronics World and the article said you orient the null-points of the
|two antennas together.
|----
|D. Woo

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