External Indoor Antenna?

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Megahurtz

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Just bought a BCT15X.

Are there any antennas that are like the included antenna but higher gain (BNC connector).

Or a small indoor antenna that gives better gain?

It's winter now where I live and don't feel like putting up an outdoor antenna until spring. Wondering if they make a decent "on-base" or indoor antenna that will do a better job that the current one it comes with,
 

n5ims

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Just get a typical mobile antenna that uses (or is mounted on) a mag mount. Stick that mag mount onto a large filing cabinet (2 or 4 drawer type from your local resale shop should be fine) or a large steel baking sheet. Should work just fine when placed near a window.
 

popnokick

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Megahurtz- You've asked about two different types of antennas:
- Something to replace the included antenna on the BC125AT
- An external antenna that can be used indoors for better performance than the stock antenna

For mounting directly onto the BNC connector of the BC125AT (no feedline/coax) there are telescoping antennas that offer much better performance than the stock antenna (almost anything other than another "rubber duck" will do that). However, they are larger / longer. Two very popular antennas are the Diamond RH77CA and a similar design from Radio Shack. LOTS of info on both of them here on RR... use Search.

Using a mobile antenna indoors has already been suggested. But if you'd like something that offers the following features:
- No file cabinet, cookie sheet, or other ground plane / counterpoise required
- Completely portable.... rolls up easily
- Very broadband, covering from 30 mHz to about 1 gHz (all popular scanning / aero bands)
... then you should look at this antenna in the RadioReference Antenna Wiki:
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

The Off-Center Fed Dipole wire version (2nd one shown... scroll down the Wiki page) will cost about $10-$14 (depending on what parts you might already have) in parts from a Radio Shack or most home stores.... and about 20 minutes of your time to assemble and hang in an upstairs window... or better yet, your attic.

How well does it work? Don't take it from me... search here on RR using "OCFD" as your search key and read what others have said about this low-cost, high performing antenna.
 

Megahurtz

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I appreciate your reply, but you misread my model number, I have a BCT15X base scanner, not a handheld.
 

popnokick

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Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 7_0_4 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11B554a Safari/9537.53)

Sorry... Then disregard what I wrote about the antennas mounting directly to the scanner and focus on the OCFD... which is appropriate for the BCT15X.
 

LIScanner101

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I'm very intrigued about the bell wire version. It would be MUCH more portable than my MON-53 when I travel to California. However I only have two questions if you don't mind...

1. For listening to CHP (42MHz), VHF high, UHF and 800 what should the lengths of the wire be?

2. Is there a formula or something to estimate bandwidth based upon wire diameter?
 

LIScanner101

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Sorry, two more...

1. Where can you get that balun with the BNC jack?

2. Do I have to use 75 ohm wire (RG-6) or can I use 50 ohm wire (is it critical for scanning to match the impedance)?
 

popnokick

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LIScanner- (replying to your questions):

I've made several of the wire versions and one copper pipe version of the OCFD all band scanner antenna. They do receive in the 42mHz range... I have a local utility company I hear all the time at 37 mHz, and I even receive a 29 mHz ham repeater with mine. Per RR member hertzian, the coax feed line actually becomes part of the antenna when using it for VHF low band, so perhaps the more coax you use the better (up to a point of high loss, of course). And yes, the wire version is extremely portable since you can simply roll it up and put it in a bag. When you arrive, hang it vertically in the hotel room window. You could easily add a plastic shower curtain clip or other non-conducting clip to hang it from the hotel curtain rod. At about $10 to make the wire version how can you afford not to try it? The feedline coaxial cable should run away from the antenna legs at a 90 degree angle for at least 12-18 inches, so you may have to futz with the coax to achieve that near-90 as possible angle. Don't let the coax run down parallel to one of the antenna legs.

"For listening to CHP (42MHz), VHF high, UHF and 800 what should the lengths of the wire be?"
The wire lengths need to be 18 inches on one leg, and 48 inches for the other to have the antenna perform from about 30 mHz to 800 mHz, and to ensure the feed point where the 300 to 75 ohm TV transformer attaches is close to 300 ohms impedance.

"Is there a formula or something to estimate bandwidth based upon wire diameter?"
I've never seen one, and don't notice that much difference in bandwidth between the wire version and the copper pipe version. But since hertzian wrote it, I know it has a very high probability of being good science.... and he has probably tested it. Recently I read a post with a novel idea to get a larger diameter element than hookup wire, yet still be much more flexible than copper pipe: Use RG-174 coax for the 18 and 48 inch legs, but solder the braid to the shield on each leg, effectively making the coax a single electrical conductor with a larger diameter... but still able to roll it up.

"Where can you get that balun with the BNC jack?"
You can't. But you can get an adapter for an "F" to BNC adapter at Radio Shack, Amazon, and even some hardware stores. Here's a link with a pic, or Google "F to BNC adapter":
F Female to BNC Male Adapter Use a length of RG-6 75 ohm coax to connect to the 75 ohm connection on the TV transformer, and connect the other end of the RG-6's F connector to the F to BNC adapter. The BNC will then connect to your radio's BNC antenna jack.

This RR thread discusses the natural resonant freq of the OCFD, low band coverage, the need to feed it with 75 ohm coax, and VHF low band coverage:
http://forums.radioreference.com/build-your-own-antenna/267596-homebrewed-off-center-fed-dipole-question.html

If you read all you can about the OCFD using the RR Search function, you'll find a lot more info, including a thread with antenna lobe plots at various frequencies for the OCFD. I think I've seen a pic of your hotel room setup here on RR. If you try the OCFD, please send a pic and let us know how it works for you!
 

LIScanner101

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popnokick,

Thanks for that OCFD link. I think I have read it before but it was good to see it again to refresh my thoughts.

I am a little confused about your answer to me about where I can get the BNC jack with two eyelets that is pictured in the link. When you say "you can't", do you mean that it's not made anymore, or that it's not actually a balun? My concern is: exactly how do I connect the two wires to my scanner? Assume that I'll be taping the wires to the inside of a hotel window and will run a length of coax (probably about 20') to my scanner which will be sitting on a nightstand. I assume that it has to be 75 ohm coax but if it doesn't please say so ;) . Thanks.
 

popnokick

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I understand your confusion now! The "BNC jack with two eyelets that is pictured in the link" is not a BNC jack at all.... it IS actually the 300 to 75 ohm TV balun transformer! Looking at it now it does appear to have a BNC connector.... but I've never seen one with BNC. Normally, they are available with a bayonet F male connector intended to slip onto the back of a TV set's F female connector. The two screw binding posts are used to connect a "rabbit ears" antenna. But there are many form factors for a 300 to 75 ohm TV transformer; just Google "300 to 75 ohm TV transformer" and click on "Images" when you get the search results and you'll see what I mean.
Using an F male bayonet type transformer with two binding posts/screws will permit you to attach the wire elements of the antenna directly to the transformer. Insert an F double female into the transformer's F male bayonet. You'll then use a standard piece of 75 ohm TV coax (whatever length you choose of RG6 or RG59) with an F male connector at each end. At the scanner end, attach an F female to BNC male adapter. And there are all sorts of variations... you see them when you go shopping online or at Radio Shack, home hardware store, etc.
For your purposes, you probably want something as small as possible. The Radio Shack 15-1253 is one possibility, but they are available in many forms. Here's one on Amazon for 40 cents (and $14 shipping!... you can do better than that):
Amazon.com: 75 To 300 Ohms Matching Transformer: Electronics
 

kb2ztx

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Actually the one in the picture may have a BNC connector. I built 2 of these also and swapped out the factory connector for a BNC panel mount jack. Just easier to snap on a BNC cable than a F-Connector for me and they stay attached ! Either way will work fine, I just liked the BNC on it instead.
 

popnokick

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I travel a lot and I think LIScanner does too. For that reason I avoid BNC connectors... or at least keep it to a minimum of an F to BNC adapter. I know I can walk into any Home Depot, Lowe's, or scores of other similar stores and get multiple types of F connectors, cables, extensions, crimping tools, strippers, adapters, etc. Regardless of where I might be. BNC connectors? Not so much.... Or even, "Never seen one like that before..."
 

kb2ztx

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Agree. Guess as a radio geek I have used BNC for years. Everything I own has a BNC or N on it. I guess its all a preference of the user.
 

KevinC

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Actually the one in the picture may have a BNC connector. I built 2 of these also and swapped out the factory connector for a BNC panel mount jack. Just easier to snap on a BNC cable than a F-Connector for me and they stay attached ! Either way will work fine, I just liked the BNC on it instead.
I bet the one in the Wiki article has one of these...

BNC Female to F Female Adapter
 

LIScanner101

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Thank you again!

The connector in that photo really does look like a BNC jack, which is what threw me off. I tried Googling it but came up dry each time - now I know why LOL.

If I use this style balun is it still necessary to keep the first foot or so of coax perpendicular to both vertical wires?
 

popnokick

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The "textbook" answer to the need to keep the feedline coax running perpendicularly away from the antenna elements for 12-18 inches is that it preserves the feedpoint impedance and the balanced directivity of the antenna's pattern. HOWEVER, in the portable situation where you have the wire version of the OCFD hanging in a window from a curtain rod it would certainly be acceptable to experiment with the position of the feedline relative to the antenna elements to see what effect it has on receiving. You certainly can't harm the receiver by doing so, and you may find that moving the feedline (e.g. letting it droop so as no longer 90 degrees to the elements) causes an increase in strength from the transmitter you are trying to receive.
 

LIScanner101

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Actually the one in the picture may have a BNC connector. I built 2 of these also and swapped out the factory connector for a BNC panel mount jack. Just easier to snap on a BNC cable than a F-Connector for me and they stay attached ! Either way will work fine, I just liked the BNC on it instead.
May I ask how you did that? I'm pretty handy with electronics (I have some decent small tools and my soldering skills are "excellent" :lol: ) so I would prefer to put on a BNC jack too. What model/brand of balun did you use that allowed you to do the swap-put?
 

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LIScanner101

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hertzian

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Easiest way for the ocfd - especially good for testing prior to committment with a sturdier pipe-build:

Radio Shack #15-1253 300:75 ohm transformer. (See below for better isolated transformer). 18 inches of wire (gauge not critical) to one of the screws. 48 inches to the other screw. Use a female - female F connector. A small run of say 12 foot pre-connectorized RG-6 to scanner. Will need an F-to-BNC adapter at the scanner. Hang vertically, and ideally run the coax away from the wires horizontally for a few feet.

Antenna is broadband in matching, but actually varies from 100 to 400 ohms or so depending on frequency. Transformer lowers the impedance to get somewhat near 50-75 ohms plus or minus, and definitely not perfect, but doesn't need to be for general purpose duties. So exact impedance of coax used is not critical. Previous descriptions of project show that due to construction, while it has a relatively good impedance matching capability (although not ideal), the look-angles get very high angle at UHF and above.

This may or may not be an impediment when indoors, or in cramped conditions where you don't have a good line of sight anyway, and may depend on reflections instead of perfect vertical polarization especially at UHF or above.

This is a compromise antenna that needs empirical testing at your site to see if it picks up what you want, as compared to a more specialized antenna. In other words, don't overthink or go nuts trying to optimize what can't really be optimized. You can however experiment with tilted or other odd mounting / hanging schemes since the reception lobes are not perfect to start with and never will be. It is however a HUGE improvement over a rubber duck usually. Note that you will NEVER see one of these in commercial service.

You can use anything to hang the thing vertically, even taping it to glass when traveling with nothing more than blue painter's-tape for a week or so. Tape it to a section of pvc if you like, but still you have to run the coax away from it perpendicularly (horizontally more or less) for a few feet. Not recommended to tape it next to drywall or any other rf absorptive surface just like any other antenna. In this case, hanging with a thumbtack in the celing with the lightweight transformer version may be a better bet.

For a sturdier build, and a little bit better transformer since it is isolated, use a Radio Shack #15-1230 or 15-1140. This is the one with twinlead leads. You don't want to stretch the twinlead too far apart - try to keep them parallel to each other for the whole length. Use a two-position barrier strip (or whatever) for attachment to the wires. I had a 4 position #274-658 laying around and two of the positions go unused. The reason is that you don't want to spread the twinlead section too far apart as it will then become part of the antenna elements, and your wire lengths will then be of the wrong dimensions.

Of course if you commit to tubing for the elements, then a direct attachment from the transformer twinlead to the *ends* of the tubing sections is ok - the key thing to avoid is spreading the twinlead too far apart for the same reason that the tubing lengths are now the wrong length with the spread-out twinlead actually becoming part of the overall antenna. But this is NOT caliper-measured critical - common construction methods ok with a little bit of sloppiness. Try the wire versions first before building one with tubing as you may be very disappointed depending on circumstances.

Tubing diameter is also not critical - just use what gives you the necessary stiffness for your application. What fat tubing does is broaden the initial 100-400 ohm impedance swings as it crosses the spectrum, but the offset feed and it's longwire-like high angle reception lobes at uhf and above will be more critical than matching is.

Warning - with the dimensions listed (48 inches one side and 18 inches the other), the primary resonance is in the FM broadcast band, so FM broadcaster overload *could* become an issue. We merely offset-feed (note that this is NOT a classical 1/3 offset, but tweaked) this halfwave to achieve wideband matching, but the reception lobes suffer in a purist environment. But in a poor location, this may not matter and the end-user will have to test it to give it a big thumbs-up, or immediately decide to throw it in the trash. I have done both depending on location :)
 
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