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 DreadPirate 25th March 2011 03:00 PM

DIY FM Antenna

This self made yagi looks like something I could handle:

The design is centered on 100Mhz. What modifications would I need to make to center it on 90.9? I'm in the DC area and this is the classical, it's also close to 89.3, the jazz station, the only stations I really care about.

 geraldfryjr 25th March 2011 03:30 PM

Just recalculate the dimensions as shown in the pictorial for the center frequency of your choice. jer

 benb 25th March 2011 07:41 PM

Or increase all dimensions (distances) by ten percent.

 Johnny2Bad 25th March 2011 08:56 PM

You want the centre frequency to be a bit higher than the midpoint of the band you're interested in ... note here I mean broadcast band not a specific frequency.

So, FM, 88~108 = mid frequency of 98 MHz. Because of what will follow, most are actually tuned a little high ... say 100 Mhz. Coincidence?

You could tune the antennae to your favourite channel (89.3) ... the math is simple and there are online calculators you can use.

However, I'm going to suggest you don't do that. Like all radio, the broadcast power is much higher at the bottom of the band than the top ... that is why radio stations are always fighting to go lower in the band if there is a change in the local market that frees up a space down low.

For them it's economics ... a 10,000 watt transmitter may cover many times the potential broadcast area at 88 than at 108 ... in fact they may need a 100,000 watt transmitter to equal the 88 range if they're at 108 (numbers pulled from my a**, but they're not completely ridiculous examples).

For you, that means any reasonable FM antenna will have no problem nailing 89.3 with strong signal strength. Tuning a DIY antennae to your desired frequency ... which are often much better than commercial designs at the low range of price ... may actually overload your tuner's front end. Also, there is a potential problem with interference ... your desired frequency would make an excellent antenna for 78 Mhz as well ... right in the old analog broadcast TV band, used by who-knows-who now that they auctioned off those frequencies.

I did quite a bit of research on FM antennas for a vehicle project a while back. The big questions regarding which design to use really boll down to where it will have to be, and to what extent you can install it.

Let us know what you're thinking there ... does it have to be in the same room as the receiver, can it mount on a roof, do you live in an apartment, that kind of thing. Can you "see" the transmitter you are interested in (line of sight)?

Actually building an antenna is quite easy. The math is hard, but there are plenty of computer-friendly tools to help there.

You want a good, fundamentally sound antenna that is not actively amplified ... they tend to have high distortion versus a properly sized one.

I'll give you a hot tip that you won't find without digging very, very deep into the documentation, most of which is written for HAM enthousiasts anyway. The standard dipole antennae ... it's always shown in diagrams, even in the comprehensive textbooks, as "T" shaped so everyone assumes it has to be that shape.

Wrong. Make it any shape you want, as long as both sides are identical (mirror images). "U" shape ... no problem. Almost closed "O" shape? No problem. Long and thin, narrow and tall, whatever. Fold it to whatever simple shape ... 2 or 3 bends ... you need, to fit wherever you need to put it. Make a dipole around 57" long, or any fraction or multiple of that length for a basic working FM antenna.

That YAGI you're looking at will be extremely directional ... you will need to be able to mount it in a more-or-less permanent location, as high as possible, and aim it at the transmitter you want to receive. It will reject stations at a fairly high ratio it's not aimed at, and eat any wanted or unwanted signals in the same direction it's aimed at. The modifications to alter the tuned centre frequency down a bit can be summed up simply as "making it shorter from the math you punch in to an equation". But, since it's a multi-element antenna with some moderately complex interactions, there will be a bunch of dimension changes you will need to do.

 7n7is 25th March 2011 09:29 PM

Here's mine.
getting WMUC

 stratus46 25th March 2011 09:44 PM

IMO the little bit improvement you gain by changing the antenna dimensions will be insufficient - if it's even detectable. Preamps won't do much for you either unless it's a rather long downlead (>>100feet). Antenna height has a much greater effect. More elements (higher gain) might help some also but height is the biggest variable.

Good luck

 7n7is 25th March 2011 10:08 PM

I found that preamps and distribution amps make a big difference for me for both FM and TV. Besides using a preamp with my FM antenna (it worked good with TV, too), I have cables going to other rooms which are connected to my VCR and a couple TV sets and they need a distribution amp to keep the noise level low enough.

 Speedskater 25th March 2011 10:28 PM

I just posted this in the other antenna thread.

For tons of stuff on FM antennas, both store bought and home-made see:

88–108 MHz

He also has a page on the Sony "XDR-F1HD" tuner.

 AEIOU 25th March 2011 10:31 PM

1 Attachment(s)
Here is a DIY FM antenna from AUDIO Magazine July 1989.

 Tubelab_com 26th March 2011 03:06 AM

Quote:
 What modifications would I need to make to center it on 90.9?
Quote:
 Or increase all dimensions (distances) by ten percent.
This is true. The dimensions of an antenna scale linearly with frequency.

Quote:
 The modifications to alter the tuned centre frequency down a bit can be summed up simply as "making it shorter from the math you punch in to an equation". But, since it's a multi-element antenna with some moderately complex interactions, there will be a bunch of dimension changes you will need to do.
To lower the resonant frequency of an antenna or speaker cabinet or a trombone or ANYTHING you make it bigger. In the case of a yagi antenna scale ALL dimensions up (bigger) by 10%.

Quote:
 IMO the little bit improvement you gain by changing the antenna dimensions will be insufficient - if it's even detectable.
Probably also true. Aiming and placing the antenna correctly will make a much bigger difference.

Quote:
 the broadcast power is much higher at the bottom of the band than the top
This is very wrong. 50+ years ago there was a little bit of truth to this for two different reasons. The technology available at the time made it easier for a transmitter to generate power at lower frequencies. It was also much easier to build a receiver with a low noise figure at lower frequencies. Both of these statements are still true but the threshold of difficulty now begins in the GHz region. Now almost all broadcast transmitters (radio and TV) are regulated in terms of their ERP (effective radiated power) by the FCC.

Technology exists for an FM broadcast station to operate at their maximum legal power on any frequency in the FM band. Most of the larger stations run near their legal limit if their potential listening audience is big enough. There is no reason for a radio station to bang out 50 KW to cover a small town. Areas with unusual geographical features may aim their signals to cover the targe population. Here in south Florida there is no reason to cover the Atlantic Ocean or the Everglades since there are few listeners there.

Quote:
 your desired frequency would make an excellent antenna for 78 Mhz as well
it is true that most antennas have reasonable gain outside their desired frequency range. The Yagi is one of the antenna types with the least "spillover".

Quote:
 ... right in the old analog broadcast TV band, used by who-knows-who now that they auctioned off those frequencies.
Used by the new digital broadcast TV band. The TV frequencies below 698 MHz were not touched. Frequencies above 698 MHz were auctioned off, mostly to cellular phone services, although some frequencies are still in limbo.

Low powered TV stations were given the choice to remain analog, or convert to digital. We still have a few analog TV stations on the air here. All high powered TV stations had to change over to digital transmission. They were given the choice to change the operating frequency that they use. Now the channel displayed on your TV set does NOT have any correlation with the actual frequency that the TV station transmits on. That is why a new TV needs to scan in order to build the channel map.

What happened when all the TV stations got the option to move? Did they all migrate to the lower channels? NO! In fact just the opposite happened. In South Florida (and most everywhere in the USA) the lower numbered channels are now vacant (and may eventually be refarmed). Everybody wanted to migrate to the UHF frequencies. WHY? Economics.

The maximum ERP power authorized by the FCC for TV stations is 1 MW (yes 1 MILLION watts). You can make a million watts of ERP with 100 KW transmitter and a 10 db gain antenna, or you can use a 10 KW transmitter and a 20 db gain antenna. The electric bill for a 100 KW transmitter is 10 times higher than a 10 KW transmitter. Since antennas get smaller with increased frequency it is far easier to build a 20 db gain antenna at 600 MHz (channel 36) than it is at 60 MHz (channel 2). Since you only have so much tower space a 20 db antenna for VHF can not be realized.

Quote:
 You want a good, fundamentally sound antenna that is not actively amplified ... they tend to have high distortion versus a properly sized one.
Again not neccesarily true. There can be several reasons for poor reception. Weak signal, interference, multipath, obstacles blocking the signal, and poor receiver performance. Often there is a combination of these.

If the poor reception is due to weak signal AND there are NO problems from nearby strong signals an amplifier placed at the antenna will almost always help. An amplifier placed at the receiver usually does nothing (and may make things worse) unless the receiver really sucks.

If the poor reception is due to weak signal AND there ARE strong signals in the vicinity of the receiver an amplifier may help IF it is a high quality amplifier with good IMD performance. Unfortunately most of the amplified antennas available on the market today do not fall into this category. IMD in an RF amp is mathematically the same as IMD in an audio amp. However its effects are different. The stronger signal will cause distortion on the weaker signal. In mild cases it may only be noticible when the strong signal has loud audio. In bad cases the weak signal will be totally wiped out.

If there are obstacles between you and the transmitting antenna, placement of the receiving antenna may have the biggest impact on performance.

Many new receivers (and amplified antennas) are designed and built to a price point. The performance of the first RF amplifier inside the receiver and the RF filtering used will determine the receivers ability to reject strong local signals and pick up weak ones. The RF amplifier chip found in many receivers costs under \$1 in large volumes. I used a \$10 chip in my DIY amplified TV antenna made by Hittite Microwave (the HMC616LP3). It is typically used in military radios. The million watt TV stations in Miami are 8 miles away, yet I have no problem getting TV from Fort Meyers (125 miles) and Fort Pierce (80 miles). It doesn't cover the FM broadcast band though.

Quote:
 That YAGI you're looking at will be extremely directional ... you will need to be able to mount it in a more-or-less permanent location, as high as possible, and aim it at the transmitter you want to receive.
This is 100% correct. A yagi must be pointed correctly. If the interfering signal is in a different direction than the desired signal this can be a very good thing. I got a cheap Radio Shack antenna rotator to point the antenna from inside the house. I also got my FM antenna from Radio Shack about 10 years ago and it is a 4 element Yagi much like the one in the plans. I have no idea if they still sell it. I also use an amplifier of my own design, but I made it 10 years ago. I don't remember what's in it.

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