Rabbit ears air antenna experiment leads to question about antenna length . . . help needed

KB2GOM

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Here's what it looks like.


G12 TV antenna exp 001 (Medium).JPG

G12 TV antenna exp 002 (Medium).JPG

G12 TV antenna exp 005 (Medium).JPG

Listening this AM, seems to work pretty well on my local public safety comms.
 
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KB2GOM

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Throw away the 300ohm ribbon and the transformer (4:1 balun ?). Disassemble the rabbit's ears and join the coax cable to the telescopic elements at the base. Re-assemble. Run the coax to your receiver. Set the lengths of each rod to a quarter-wave for you frequency of interest.

A 300ohm antenna is usually a loop like mine. The 300:75 balun is in the orange box . Works well on the 118-136MHZ air band.

View attachment 106566
Did you buy it or build it? Looks very professional.
 

KB2GOM

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Tuner wheel? So that implies that there are some other components in that rabbit ears antenna set (coil / cap maybe). May be helping, but might also be reducing your signal. You write that the rabbit ears give you better (stronger) result than the Comet 100, so you might benefit by following the instructions in the RR Wiki for the Homebrewed Off-Center Fed Dipole (linked previously). Use the 300 to 75 ohm transformer (or get another... they're inexpensive) and make the "wire version" shown in the Wiki. Hang it vertically in the highest window you can (or attic). Run the coax away at a 90 degree angle to the two vertical antenna elements.
See pix above.
 

popnokick

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Look again at the pics in Wiki page for an Off-Center Fed Dipole Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
Notice that the two antenna elements are in a straight line running away from each other... not in parallel as in your TV antenna pic. Your TV antenna as it's presently configured is probably closer to a J-pole... which is going to make it very narrow in frequency coverage. So you have two options: 1) Get a new 300 to 75 ohm transformer and some wire for about $15 at any hardware store or Walmart and make the OCFD as described on the RR Wiki page; or, 2) as majoco suggested, make a dipole tuned for the aircraft band by ditching the 300 to 75 ohm transformer entirely and connecting the coax directly to the antenna elements. HOWEVER, you will still need to have each leg of the dipole vertical and running away from the other at 180 degrees, not in parallel to each other. Based upon the pic of your existing TV antenna it doesn't appear that is possible.
The advantage to option 1 (OCFD) is that it will be useful across a wide swath of frequencies that can be received by your scanner. The dipole is going to operate best in the civil air (118-136 mHz) band, and may give you some reception on other frequencies but not as efficiently as the OCFD.
 

KB2GOM

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Look again at the pics in Wiki page for an Off-Center Fed Dipole Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
Notice that the two antenna elements are in a straight line running away from each other... not in parallel as in your TV antenna pic. Your TV antenna as it's presently configured is probably closer to a J-pole... which is going to make it very narrow in frequency coverage. So you have two options: 1) Get a new 300 to 75 ohm transformer and some wire for about $15 at any hardware store or Walmart and make the OCFD as described on the RR Wiki page; or, 2) as majoco suggested, make a dipole tuned for the aircraft band by ditching the 300 to 75 ohm transformer entirely and connecting the coax directly to the antenna elements. HOWEVER, you will still need to have each leg of the dipole vertical and running away from the other at 180 degrees, not in parallel to each other. Based upon the pic of your existing TV antenna it doesn't appear that is possible.
The advantage to option 1 (OCFD) is that it will be useful across a wide swath of frequencies that can be received by your scanner. The dipole is going to operate best in the civil air (118-136 mHz) band, and may give you some reception on other frequencies but not as efficiently as the OCFD.
Popnokick,

The wire version of the homebrewed off-center fed dipole sounds like the way to go.

For nearly 5 decades I wrote about science, tech, and medicine from a layman's point of view, and my "big trick" was being unafraid to admit my ignorance.

Sooooo . . . when I go to the hardware store,

- what sort of wire should I buy?
- what, specifically, should I ask for when I want the transformer (I plan on calling before I go to the store)
- and the lugs . . . just ask for lugs?

For a feedline, a plan to use the one that is currently connected to the rabbit ears antenna.

BTW, thanks for your help!
 

popnokick

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Popnokick,

The wire version of the homebrewed off-center fed dipole sounds like the way to go.

For nearly 5 decades I wrote about science, tech, and medicine from a layman's point of view, and my "big trick" was being unafraid to admit my ignorance.

Sooooo . . . when I go to the hardware store,

- what sort of wire should I buy?
- what, specifically, should I ask for when I want the transformer (I plan on calling before I go to the store)
- and the lugs . . . just ask for lugs?

For a feedline, a plan to use the one that is currently connected to the rabbit ears antenna.

BTW, thanks for your help!
Wire - Any wire will work. "Bell" wire, THHN, automotive primary wire, etc. If you go to one of hardware chain stores (Home Depot, Lowes, etc.) just go over to the "wire cutting area" in Electrical. Typically they have a bunch of random lengths lying around of various types of single conductor wire in the cutting area. (Two-conductor wire is OK too, but short the ends together when you connect so it all is electrically a single wire.) In a box or bin near the cutting area they'll have those random cut pieces wrapped in tape with a price on them. You need at least 66 inches... buy a little more in case you make a mistake measuring.
Transformer - They come in multiple form factors. Here are two examples from Home Depot (Lowes and others have them, too) that are better suited to the copper pipe version of the OCFD -
Search Results for tv transformer at The Home Depot
If you're making the wire version you'll want a transformer that is more like this because it has screw lugs for the wire -
Amazon.com: Fancasee (2 Pack) 300 Ohm to 75 Ohm UHF VHF FM Matching Transformer Balun Converter Adapter with F Type Male Coax Coaxial Connector Plug for Antenna Cable Cord TV: Industrial & Scientific
 

KB2GOM

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for me, the ATIS signal was the hardest to catch, I was receiving lots of airband traffic, got the antenna 28ft up in the air and am able to receive the local ATIS , which is less than 5 miles away as the crow flies
I've noticed that my local ATIS signal varies somewhat in strength, listening with the same radio, same antenna, same location, etc. . . . I don't know why.
 

PDXh0b0

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this is the thread I landed on when first trying to receive our ATIS
 

KB2GOM

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this is the thread I landed on when first trying to receive our ATIS
Google maps tells me I am 7.8 miles from Albany (NY) International Airport. The "Amazing Color Supreme II" rabbit ears antenna brings in ATIS at two bars on the signal strength meter.

I am in the act of assembling the stuff for the wire version of the off-center fed dipole. We'll see how that works.
 

KB2GOM

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Well, with the help of a ham friend (thanks again, Steve!), I built the Homebrewed Off-Center Fed Dipole - The RadioReference Wiki (OCFD).

Wire: I used some white insulation multistrand I found in the workshop.
300-75 ohm transformer: Amazon, two for less than $8 (apparently, you can't buy just one).
Lugs: local hardware store insulated lugs.

The only glitch was that the lugs I bought would not fit over the insulated wire on the free ends of the dipole. Fortunately, Steve had some larger lugs, and with those in hand, assembly took only five minutes. So, note well: if you plan to build the wire version of this antenna, make sure that you have some lugs that are big enough to fit over the insulated wire.

I hung the completed off-center-fed dipole vertically inside in my radio room (discretely, along the edge of a window frame) and hooked it up to the SDS200. I can't prove, based on the signal strength meter, that the OCFD outperforms the rabbit ears antenna when receiving ATIS from the Albany County Airport, but it is certainly no worse, and the OCFD clearly outperforms the metal telescopic antenna that comes with the SDS200.

And it seems like I am getting better audio from the local public safety channels I am monitoring for the Commuter Assistance Net. I really can't explain why that should be so.

Bottom line: the OCFD is definitely an experiment worth doing. It's inexpensive, easy to assemble (the wire version), and delivers worthy performance. I plan to keep mine hooked up to the SDS200. Thanks, popnokick, for recommending it.
 

KE5MC

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You might try attaching to a wood dowel rod or trim molding. Allows trying different placements which could improve reception. Metal in the window frame or metal drywall edge flashing could be messing with the results. Also various non-vertical angles might have surprising results.
Good Luck!
Mike
 

popnokick

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KB2GOM - Thx for updating us here on this thread! And glad you had success with the HomeBrewed Off-Center Fed Dipole and your scanner. There has been much written about it here on RR if you use the Search function you will find the threads and comments. It's certainly not a perfect indoor antenna but it has the following going for it -
- For $15 (or less) and 15 minutes (or less) it is very inexpensive to try
- For the price and time spent it has a very high return on value
- As you note, it works better than the back-of-set telescopic or "rubber duck" antennas that come with most scanners
- It is ideal for use indoors and is easily concealed behind window curtains, non-metallic blinds, etc.
- It has a very wide frequency coverage. Mine receives on the lower end of the range plenty of CB from a nearby interstate (27 mHz), the local electric utility in the VHF Low Band, 6 Meter FM Amateur Radio, up through the VHF air band (as you've discovered), VHF High Band (including state police base transmission one state over and more than 75 miles away), military air band (including AR, CAP, and other operations I know are 150 - 200 miles distant, as well as the P25 frequency of a military post about 15 miles from me), the 70cM ham band, and the UHF Public Service band (450-512 mHz). I haven't tested it extensively in 700 or 800 mHz... it's been written on the forums here that at those freqs there are sharp receiving lobes, most of them skyward. But it does receive there.
- Unlike a dipole or discone, the OCFD offers some gain at some frequencies
- It's extremely portable and easily moved to different places in a house / building, and even rolled up and thrown into a suitcase for travel.
 

KQA726

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for the HOCFD antenna, what is the frequency range (bandwidth) for the 48" x 18" model using 1/2" or 3/4" copper pipe? And you DO paint the pipe, no bare copper?
 

popnokick

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for the HOCFD antenna, what is the frequency range (bandwidth) for the 48" x 18" model using 1/2" or 3/4" copper pipe? And you DO paint the pipe, no bare copper?
What I have seen written here on RR... but no test data produced or shown... is that the larger diameter of the elements (copper pipe) will produce a greater bandwidth in each of the resonant "sweet spots" of the OCFD antenna. There's a project for someone (comparing bandwidth of various copper pipe diameters on the OCFD). There is no requirement to paint the pipe... mine is bare copper. But it is in the attic so I have no need to paint it. However, it could be painted with any non-metallic content paint once it was assembled and all the electrical connections secured. There are MANY threads here on RR regarding painting antennas and what paint to use. Search is your friend.
 

kny2xb

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Someone here on RR posted that they had repurposed an old TV rabbit ears antenna for monitoring air frequencies and that it worked pretty well (If I could find the post again, I would reference it). If I recall correctly, they adjusted the rabbit ears until each was about 18 inches long. I look up online, and a quarter-wave of 128 MHz is around 19 inches. Close enough.

Hmm, I thought, I've got an old rabbit ears antenna somewhere . . . I found it; a ham friend (thanks, Steve!) whipped up a jumper cable to connect the rabbit ears to the BNC connector on my SDS200, and -- voila! -- we're in business. I figure the ATIS signal at Albany Country Airport will work just fine as a reference signal, so I lock the SDS200 onto that. The indoor antenna I am using for the SDS200 is a Comet W100RX, extended to its full length, about 40 inches. The signal strength meter is showing just one bar.

I hook up the rabbit ears (set to 18 inches long) to the SDS200, and the ATIS signal is immediately worse; no bars. Making sure the switch on the antenna is set to VHF, I try clicking the dial for various VHF settings on the rabbit ears antenna. No improvement, and some positions are noticeably worse.

As I am putting the Comet antenna back on the SDS200, I notice that there are markings on the black barrel of the first segment of the telescopic antenna. One of them says "VAIR," and it indicates that the W100RX should be extended 5 1/2 segments to receive VHF air frequencies. So I try that and -- ta da! -- the SDS200 shows two bars on the ATIS signal. Cool! I measure the length of the Comet antenna at 5 1/2 segments, and it's 27 inches to the right-angle bend where the BNC is connected.

So maybe the rabbit ears need to be adjusted to 27 inches. I try that, and it works no better than before.

That 27 inches looks kinda familiar, though, so I measure the stock telescopic antenna that comes with the SDS200, and it is almost exactly 27 inches. I connect that antenna to my Uniden 125AT, and it, too, shows two bars of signal strength on the ATIS frequency (and that's better than my Diamond 77 antenna). Very nice . . . an improved setup for listening to civilian air frequencies on two scanners.

But now here is the question: why should a 27-inch antenna work better? It's not -- as near as I can tell -- quarter wave, or half wave or five eighths wave. It is approximately 3/8 wave.

Can someone please shed some light?
That was me, I posted that way back in March, 2006 Indoor Scanner Antenna

I had simple rabbit ears like this: https://i.ebayimg.com/images/i/161269129068-0-1/s-l1000.jpg,
not the high end model like yours (I remember that one from when I worked at Radio Shack, wow, the Archer brand), I was using the ones that came with my TV

I was trying to DIY the Grove Enterprises indoor antenna, so I extended each whip to 33" since the total length of the Grove was 66"

I didn't know anything about antenna theory yet, so I wasn't looking at quarter-wave or half-wave, etc

I was just trying to improve my reception on a very tight budget
 

KB2GOM

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That was me, I posted that way back in March, 2006 Indoor Scanner Antenna

I had simple rabbit ears like this: https://i.ebayimg.com/images/i/161269129068-0-1/s-l1000.jpg,
not the high end model like yours (I remember that one from when I worked at Radio Shack, wow, the Archer brand), I was using the ones that came with my TV

I was trying to DIY the Grove Enterprises indoor antenna, so I extended each whip to 33" since the total length of the Grove was 66"

I didn't know anything about antenna theory yet, so I wasn't looking at quarter-wave or half-wave, etc

I was just trying to improve my reception on a very tight budget
Greg,

Your experiment with "rabbit ears antennas" inspired me to try mine.

My rabbit ears antenna sits next to the OCFD as my backup antenna, thanks in part to you. It works pretty darn well.
 
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