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Improve an FM antenna?

Posted by bluewhale (My Page) on
Sat, Apr 9, 05 at 23:23

I have an old under-cabinet GE AM/FM radio that has been having more and more trouble receiving the station I wish to listen to. I took the radio apart hoping to be able to extend whatever internal antenna was there to improve reception. I think I found the FM antenna: a yellow medium size insulated wire which went across the chassis then was tightly coil wrapped around the power cord inside the radio case. I unwound the wire and sure enough I now have lost all FM reception.
I tore radios and TV's apart as a kid and could swear the FM antenna's are supposed to be fairly straight affairs? My wild guess is the coiled wire was using the entire power cord as its antenna. Does this sound correct to the radio wizards out there? If so, or even if not how might I rig up an improved antenna for FM reception here?

Paul


Follow-Up Postings:

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RE: Improve an FM antenna?

Yes, many FM table radios without pull-up "whip" type antennas do use the power cord as the antenna. It can work okay if the power cord is extended out fairly far and straight. It won't work well at all if the power cord is left coiled up or is not extended out fully.

The simplest thing you could do, and it might work fairly well if the FM stations are not too far away, is to use a straight piece of wire about 57" long as your antenna. This is the proper length for a "half-wave" FM antenna. You would want to extend the wire out fully, and ideally you want it to be perpendicular to the direction of the station you are trying to pick up. So, if the station is directly north or south of you, you would want the wire running east-west. Since it's an under cabinet radio, possibly a convenient and fairly unobtrusive way to do it would be to tape the antenna wire underneath the cabinets.

Of course, be careful not to connect any antenna wire to anything inside the radio that could be electrically hot. Some of those old table radios use full line voltage on the circuits. Use a voltmeter to check if you're not sure.


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RE: Improve an FM antenna?

I was hoping to avoid something long, but if I have to... I had thought to drop a wire out the kitchen plant window, drill a hole in the base and drop a wire straight down. two questions arise tho:

1: did you mean 57 inches, or really mean 57 feet?
2: If I drop this wire any thoughts on whether it should be insulated with respect to performance and also safety?

BTW the unit is about 20 years old, a typical radio to mount under your kitchen cabinets. As space is at a premium I simply let it sit in the bottom of the plant window. ( I'm not sure what to call the extended window over the kitchen sink which is meant to house plants )

Paul


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RE: Improve an FM antenna?

There are normally two antennas. One for AM that is on a black looking rod with a lots of coiled copper wire. The other the FM is usually running either close to the AC wire, wrapped a bit, or around the inside of the radio chassis.
You should be able to extend this wire by soldering onto it a longer wire of several feet (insulated). Make sure that you insulate wit electrical tape where you soldered the wires together. Normally an FM should have a large loop with one side connected to this wire and the other end connected to the other end of a twin or a ground. See if there is a place to connect a 300 Ohm like external antenna.


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RE: Improve an FM antenna?

Paul, I did mean 57 inches -- just under 5 feet. That's the correct length for an FM antenna. Other lengths of wire will work, but the 57 inches puts you at the "half-wave" size of the actual FM wavelength, which increases its effectiveness.

Yes, insulated wire is best. Any type of copper small gauge wire available at any hardware store such as #20 or #22 would be fine.

You can run the antenna straight up and down, as it would be if you dropped it out a window, but usually they work better if you run them horizontally rather than vertically. The antenna does not really have to be run outdoors, although it's fine if you do that. With these wire-type FM antennas, the best reception is usually just going to come from experimentation. Try dropping it out the window, as well as running it horizontally under the cabinets, and just see which way works best.


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RE: Improve an FM antenna?

Thanks for the info. And Deer180, no there are no openings for an add on antenna: that's what I had originally hoped for. OTOH now I get to play with my dremel kit
I had not considered running the wire under a cabinet: time to experiment a bit.

Thanks again!

Paul


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RE: Improve an FM antenna?

I have been a practicing Amatuer Radio Operator (Ham Radio) for nearly 40 years and my principle interest in the Radio hobby throughout that time has been the design and construction of antenna's for Broadcast AM&FM, CB radios (27mhz band) and the full range of ham radio from 1.6mhz shortwave to 1296mhz microwave bands.

Basically all antennas fall into one of five classes,1. Vertical Marconi(single vertical element) end fed hertz (Single horizontal element) Dipole(two horizontally opposed elements from a common feedpoint), YAGI ( A variation of the dipole that has additional horizontal elements arranged as reflector or directors) or Parabolic dish,(used to reflect multiple microwave signals to a common feedpoint).

In order to achieve peak resonance the antenna element must be cut as near as possible to the 1/2 wave of the desired frequency.

All radiated electromagnetic energy travels through free space at approximately the speed of light (186,000 miles per second or 300 million meters per second.) To find resonance we must compute the distance the signal travels on ONE IMPULSE at the transmitted frequency.

In the USA the FM Broadcast Radio band is 87.5mhz to 108mhz.

When designing an antenna to work over a range of frequencies we must first find the center of the desired range, thus for FM broadcast its 87.5 + 108 = 195.5 /2= 97.75mhz.

If the signal travels 300 million meters per second, and the signal is alternating at 97.5 mhz we can then compute the distance the signal travels on one impulse-- 300m/mtrs/97.7mhz= 3.07 meters per implulse.

To convert to feet: 3.07 meters x 39"/mtr = 119.73" /12"/ft =9.975ft.

Thus one wave equals 9.75ft therefore 1/2wave = 975/2= 4.875ft. .875 x 12 = 10.5inches

Thus 1/2 wave at the midpoint of the FM braodcast band equals 4' 10 1/2"

Now take a common piece of 300ohm TV flat lead and split the ends back so you have two pieces 4' 10 1/2".

secure the ends to a wall with a thumb tack and use the remeing wire from the center point to the radio receive. You have now constructed a perfect 1/2wave dipole for the USA FM Broadcast band.

__________________ ____________________
4'10 1/2" each way from the center point


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RE: Improve an FM antenna?

Hi Lazypup:

If I have one connector on the circuit board to solder to and I run a lead out to this antenna...

1: wouldn't the 6-18" of lead count as part of the antenna?
2: If I run it 58.5" BOTH ways would that not constitute one full wavelength antenna vs the half wavelength one cowboyind and yourself suggest?

Tks

Paul


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RE: Improve an FM antenna?

OOOOOoooooh boy,,i was afraid this question would come up. It would be very easy to illustrate it on a blackboard but I am not certain how successful I can be attempting to illustrate it with the written word, but here goes. Radio energy is properly called 'Electromagnetic energy"

When the signal is paasing on a balanced feedline (coax or flat twin lead TV tape) The polarity is constantly changing at the radio frequency. So while most people think of the inner wire in a coax as the postive and the outer covering as the negative, in truth the polarity of each wire is constantly changing polarity millions of times per second. At any given instant in time, when the polarity of one conductor is postive, the oposite conductor is negative, thus the two cancel each other out or is properly called self shieilding.

In order to explain it i will describe what happens when a signal is transmitted.

The radio transmitter produces the signal, which is then fed into a coalxial cable. As long as the signal is in the coaxial cable the negative impulsule on one wire cancells the postivie impulse on the other and the current flows through the cable in balance.

Now consider a dipole antenna. The coax it brought to a center insulator. The center wire of the coax is connected to a 1/4 wave lenght wire in one direction away from the feed point, and the outer covering of the coax is connected to a 1/4wavelenth piece of wire suspened in the opposite direction.

The signal comes up the coax constantly in balance until it reaches the center feed point. The positive energy then travels out to the right hand on one wire while the negative simultaneously travels out on the left hand side. Each impulse travels to the tip of the wire and reflects back toward the feedpoint. During this phase the antenna wire is magnetized, positive on one side, negative on the other.

If the lenth of the wire is cut precisely corrct, the reflected impulse returns to the center feedpoint at precisely the same time that its opposite counterpart returns to the feed point and the antenna is in balance. However, at that same moment in time, the next impulse coming up the wire collides with the signal on the wire. Howver, the new incoming signal is at the opposite polarity of the signal still remaining on the wire. The signals both on the antenna wire and coming in from the coax repell each other, thus the signal on the wire is propelled into space as a radiated signal to allow space for the new signal to come into the antenna.

If the antenna elements are too long the signal does not return to the feedpoint at precisely the correct time to collide with the new impulse and the antenna does not radiate. If the element it too short, the signal returns to the feed point too soon, then starts traveling back down the coax, Which is referred to as a "Standing Wave" on the feedline. (This s a very serious problem that can result in burning up a transmitter.) (For those who have played with CB radio's you may be familiar with the term Standing wave Ratio or SWR- It is a measurememt of how much energy you feed an antenne versus how much is reflected to the radio. If the antenna elements are cut correctly it should radiate 100% of the energy fed to the antenna. For some radios such as my 1500 watt High Frequeincy transmitter a small mismatch of 5% could be enough to melt the final tubes in the transmiter in only three or four minutes, and considering that a set of four matched final tubes now costs $430 you can be sure i constantly monitor my antenna match.)

The only energy present on a receiver antenna is the radiated signal energy that was sent from the transmitter. While a transmitter may be sending out thousands of watts of energy, that energy is being sent in every conceiveable direction at the same time, therefore the amount of energy that your reciever can capture is extremely small by comparison. In a real good signal area you antenna may only capture a few milliononths of a watt. The receiver antenna is also contantly being bombarded by all the other radio signals in the atmosphere, The FM station you desire + all the other FM stations, AM Broadcast, TV broadcast, Police,fire, municipal services, military signals, Ham radio, CB radio, CELL phones, and all other forms of wireless communications. The receiver must then sort a miniscule little milliononth of a watt of energy out of the hundreds of watts of extraneous energy. Its sort of like sitting in the cheap seats and trying to get the lead singers phone number at a rock concert.

Fortunately for radio communications if the receive antenna is cut to precisely the correct length it can capture the signals at the desired frequency while all the signals that are tooo long or too short go by unnoticed. So you can gain some idea as to why it is imperitive to cut the receive element to exactly the correct lenght.


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RE: Improve an FM antenna?

ok.
You're right. We need a blackboard
Most if not all of what you say makes sense. However I'm not sure it addressed my question: I have ONE place to connect the antenna on the circuit board, not two. I plan on using a length of shielded wiring similar to what's in use currently, only this new antenna will go through the chassis and hang or be nailed up to provide reception. With this single wire I can measure 58.5" and solder it on. However with your two wire flat cable example, if I were to solder BOTH leads on it to the single point of connection on the circuit board wouldn't ... the positive side cancels the negative side... how would the signal know AT the point of connection on the circuit board which way to send which charge?
Actually you've helped more than enough! Just curious as I have a small transmitter antenna to erect some day and am trying to learn all I can before deploying it.

Thanks for all the background info!

Paul


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RE: Improve an FM antenna?

In any situation where there is a physical distance between the transmitter or receiver radio and the actual antenna, such as when roof mounting an antenna, you must have a balanced feed line between the two devices.

In this case you desire to connect the antenna directly to the radio receiver so you have no need of a shielded feed line.

For the type of antenna you are trying to create here, simple select an appropriate lenght of insulated single conductor wire and solder on end to the radio tuner input. Thats all you need. The type or size of the wire is none critical as there is no complete path for current in the wire, hence no heat buildup to worry about.

When designing transmit antennas there are other paramaters that must be considered when selecting the size of the wire, such as anticipated band width, whether or not the antenna will use loading coils to physically shorten the length etc but most often the real concern is, will the wire be strong enough to support itself without strething in a heavy windload. In years past, while operating my Ham Radio's in apartments that did not permit visible antennas I actually used an old transformer to get an insulated single conductor wire that was about .3mm thick to build a forty meter dipole that was 120feet long and handled 1000 watts transmit power. The wire was so fine that it could not be seen from 25feet away.

On another occassion i used aluminum rain gutter with a 6" section of vinyl rain gutter to separate it at a feedpoint. I used that rain gutter antenna for 5 years, and the whole time not a soul in my neighborhood suspected it was really a transmitter antenna.


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RE: Improve an FM antenna?

If you have an outside tv antenna, you can probably bring a portion of this lead 'close' to the radio. This will increase the signal to the set.

The fm band is located about midway in the low vhf band.

There are co-ax and 300 ohm 'splitters' that have uhf/vhf/fm terminals for connecting to all equipment.

This way, you don't need any calculations or a black board. The freq and wave length has been calculated a gzillion times.

Good luck,

pw


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RE: Improve an FM antenna?

Thanks for the thought PW. unfortunately I don't even have cable, much less a real TV antenna. Come to think of it I do have a pair of souped up rabbit ears in the garage I use for things like the superbowl... But for this radio I think just running a 58" wire out the side in the kitchen window will do. Or I hope it will. :)


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RE: Improve an FM antenna?

can I use a belden 22 gauge, sheilded, twisted wire as a lead to my dipole fm antenna? and if so, should I ground the sheild?


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RE: Improve an FM antenna?

"When designing an antenna to work over a range of frequencies we must first find the center of the desired range, thus for FM broadcast its 87.5 + 108 = 195.5 /2= 97.75mhz."

The geometric mean is used for antenna design, SQRT(87.5*108) = 97.21 MHz.


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RE: Improve an FM antenna?

Is there any way to use the dipole technology if my amplifier only has a single wire FM antenna connection? Can I use some type of convertor to go from the 300 ohm twin lead connector wire to a single wire, or does this defeat the whole purpose of the dipole half-wave principle? Any help will be appreciated.
Thanks,
Brian


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RE: Improve an FM antenna?

Out in the Great Basin of Utah and Nevada there are locations where the FM on a car radio will work but most, in my experience none of the small household FMs will not pick up anything.

What makes the car radio work?

Could this be used by persons attempting to wire up their home FM radios?


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RE: Improve an FM antenna?

I suspect it's a combination of factors that make the car radio receive FM when most home radios won't. First, the car radio is generally better quality than the average home radio. They make them to work in a difficult environment, and to last the life of the car, and usually they do. (Most cars in junkyards have perfectly good radios in them, unless someone has already taken the radio out.) A lot of home radios are $20 items that have poor sensitivity and selectivity. Second, a car radio has an outdoor antenna, while most home radios have either a relatively poor line cord antenna, or if they have a pull up "whip" antenna, it's indoors because it's attached to the radio.

You can buy home radios that receive stations as well as, or better than, a car radio. One I'd suggest if anyone wants excellent AM and FM reception is the Grundig G5. It's also a shortwave radio, but it's worth the price of $149 or so just for the AM and FM.


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RE: Improve an FM antenna?

I have an FM Radio that only has a single insulated conductor wire coming out the back of it that is about 18 inches long or so.
I have an FM DiPole(sp) antenna that looks like a T.
The working end of the DiPole terminates to a Male BNC connector. My question is, "Can I connect this to my Radio's single conductor antenna and if so do I need an adaptor or can I just solder it to the center conductor of the BNC or the outer fitting of the BNC ? I have a drawer full of those old 75/300 ohm spitters that have the BNC on one end and flat antenna wire on the other end. Can I use one of those ? And if so which one, 75 ohm or 300 ohm.
Thanks in advance for your help.
-trent-


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RE: Improve an FM antenna?

I am facing a lots of problem finding my network of FM.I have improved some of its parts but still suffering.Please help me.


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RE: Improve an FM antenna?

"Basically all antennas fall into one of five classes..."

There are a lot more than only "five classes" of antennas.

You have omitted log periodics, spirals (backed & unbacked, end-fire and broadside, with Yagi-Uda spacing, log periodic, archimedean spirals, and some others) and a number of others types.

If you have only a single station you want to receive an antenna designed to that stations wavelength will give better performance than a broad-band (like the whole FM band) antenna.

One of the penalties for better antenna gain is increased directivity.
The antenna must be pointed at the signal source (or whatever the strongest direction from you to the source).


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