DIY FM antenna, can it be better than radioshack junk?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
'Black Satin'* paint
Oh, I forgot about the asterix... OT story... near my home, some fools spray painted "SATIN RULES" on the side of a church. I'm sure they meant to write satan rules but satin is just so funny. Alas, the graff was gone by the time I came around with a camera.
Fools:
- "Dude, I just love satin...it's so smooth!"
- "F!'n A, man! Satin rules!! Praise satin!"
 
i am very confused when researching j-poles because they are mostly used for HAM and transmitting. i want to receive signal and have no idea where this all fits in.

Antennas work equally well transmitting or receiving - i.e. a good transmitting antenna is also a good receiving antenna. Otherwise it would disobey one of the laws of thermodynamics!

From the sites I've seen
1. 300 ohm ribbon varies widely in velocity factor, its not a parameter controlled during manufacture. So calculating the right length is going to be tricky.

2. fatter conductors = wider bandwidth, which is want you want for receiving FM.

3. A j-pole is an end-fed half-wave dipole, with a quarter-wave matching stub. It can be built as an end-fed 5/8th wave, but tuning is supposedly more finicky. J-poles are nicely ground independent.

4. Velocity factor for 1/2" copper pipe is about 0.95, at 98MHz

Kind of random, but that summarises some of the stuff I've found.
 
TwoSpoons said:


Antennas work equally well transmitting or receiving - i.e. a good transmitting antenna is also a good receiving antenna. Otherwise it would disobey one of the laws of thermodynamics!

But a transmitting antenna is built to take anywhere from a few watts to two kilowatts for ham use -- so the materials might be different -- this becomes problematic for yagi's and LPDA's where you have to contend with wind and ice-loading.

Most university libraries (and county libraries in the U.S.) will have a copy of "The ARRL Antenna Handbook" -- this is a great source of theory and practice. The book costs around $39 and comes with a CD-ROM of programs which put the calculations into some easy programs (although many of these are java-scripted for the web nowadays.)

It surprises me that there have been relatively few articles on FM and AM antenna's -- a couple years back there was a nice article in EW describing how a.m. radio buffs in the UK built multiple loop antennas in order to hear bluegrass from the U.S. I think that AudioXpress and its predecessor have only had a half-dozen articles in the past 30 years or so. I have only been receiving Elektor for 4 or 5 years and would be interested to find out if they have any good FM antenna articles.
 
I don't recall the exact dimensions of my J-pole,but there's info all over the net on how to build one..
One trick I used,was to drill a hole in each of the copper pipe caps that go on top (to keep rain out,etc) and solder a nut on the inside of them,then I cut a couple short (3-inch?) pieces of "all-thread",and screwed then into the endcap+nut,and put a nut on the top to lock them inplace..
This way you can fine-tune the length of the J-pole..
(Mine was actually intended to be used with a small transmitter,but it works just as well on a reciever.)

I shot it with a coat of grey spray-primer,and it blends in with the grey house nicely,It's right outside my window,and you'd hardly notice. ;)

It's matched for 50 ohms,but works quite well on the 75ohm input of the reciever. Got a run of RG-8 going to it..
 
WBTZ is quite far from Montreal, the antenna is situated in Plattsburgh, 80KMs away. They use directional antennas, instead of non-directional like most others.

You might also want to know that the antenna is 550m higher than sea level.

More info:
http://www.fcc.gov/fcc-bin/fmq?list=0&facid=52807

I have one antenna able to receive that channel perfectly; A RadioShack Active Directional FM/VHF/UHF rabbit ear antenna. It's the only active antenna I have. All others have separated signal amps. There's a strange thing with 99.9 is that if your antenna is incorrectly placed is that you receive all the powerful channels between 90and 100MHz :94.3, 95.1, 96.9 and 99.5. Also, in my case, my CDP produces so much interference that I hear the line out of my CDP in the tuner.

The only thing is that if you listen TV with the same antenna, TVA reception will suck. Doesn't matter, it's a sensationalism channel with only poor content.

You might also want to mod your tuner's IF section:
http://www.amfmdx.net/fmdx/mods.html
and Google. the link I provided is not the best I found.
 
Antenna design is mostly a matter of mathematics. You can't fool Mother Nature. Antenna elements must be able to resonate to certain frequencies. Any element (or 1/4 wave stubs placed end to end to be a 1/2 wave antenna) is only good for a limited frequency range (fatter element is better) so to get the entire FM band one needs 3-4 separate elements. Or, you could design an antenna just for the desired transmitter's frequency.

Then, consider if its signal is vertically or horizontally polarized or has some sort of a circular polarization. This would relate to how the elements are positioned.

Next shun amplified antennas. If there is no signal you can't amplify it and most modern FM tuners are extremely sensitive to the slightest hint of a signal.

Next, find the room's "hot spot" for the desired FM station. The elements should be placed at right angles to the station but moving the antenna fore and aft or left or right even a few inches can make a big difference and the directional position may not always follow logic. I currently am using TV rabbit ears with each 1/2 extended out 26" and the antenna is nested in a ceiling fixture lamp shade. That particular spot is the best spot in the entire room for some reason and I am getting quite regular good reception of an 89.9mhz station located 98 miles away up a river valley (no high hills in the way).

Also, be aware of tropospheric bending or ducting. Depending on atmospheric conditions you may get excellent reception with a coat hanger dangled from a window and at other times even the best of outdoor antennas may not provide a useable signal.

An indoor antenna is a vexsome thing and can consume a lot of time fiddling with it as the signal strength is so low in amplitude and it gets jumbled from object reflections. Multipath distortion will always be a big factor and can cause reception distortion. And, the room will definitely have a hot spot for each station.

BTW, Radio Shack antennas are not junk. If I were you I would buy their yagi FM antenna and try positioning it around the room to find the best spot and then figure out how to make it appear as a work of modern art or a mobile dangling from the ceiling.
 
EDIT: this design is a log-periodic array -- has nice bandwidth although the gain is somewhat lumpy:

An externally hosted image should be here but it was not working when we last tested it.


here's an EDIT implementation used by a ham radio operator on 432MHz which can be repurposed for FM -- this design will give around 8.5 dB of gain with a front-back ratio of 22 to 28dB:

An externally hosted image should be here but it was not working when we last tested it.


they used copper tape for the elements and affixed them to a piece of foamcore.

for a 100MHz antenna (98Mhz to 102MHz) the element spacings are as follows:
D1 = 0.259m
D2 = 0.248
D3 = 0.238
D4 = 0.228
D5 = 0.218
D6 = 0.209
D7 = 0.200

the length of the elements are:
L1 = 1.561m
L2 = 1.495
L3 = 1.432
L4 = 1.371
L5 = 1.313
L6 = 1.258
L7 = 1.204
L8 = 1.154


I realise that this is a truly ancient thread, but the photos in this post have really interested me. I haven't been able to find any examples/instructions for a log periodic antenna which looks anything like this anywhere else, so if anybody has any more information, I'd be very grateful. I am intending to build a directional FM antenna, and I liked the idea of making a thin yagi to put on my ceiling, and then I saw this. Looks great, but the lengths mentioned seem to indicated that it will actually have to be enormous for FM. Is this right, or are the numbers wrong? Also, I can't see from the photos, but how/where does the feed cable attach?

Many thanks for any ideas!
 
I realise that this is a truly ancient thread, but the photos in this post have really interested me. I haven't been able to find any examples/instructions for a log periodic antenna which looks anything like this anywhere else, so if anybody has any more information, I'd be very grateful. I am intending to build a directional FM antenna, and I liked the idea of making a thin yagi to put on my ceiling, and then I saw this. Looks great, but the lengths mentioned seem to indicated that it will actually have to be enormous for FM. Is this right, or are the numbers wrong? Also, I can't see from the photos, but how/where does the feed cable attach?

Many thanks for any ideas!

Build a Yagi for FM, especially for a first project. IMO, nothing can touch a well built, purpose designed yagi for FM. Anything you build based on a good design is going to be light years ahead of and outperform anything you can buy or in the past.

You can see mine on the 88–108 MHz website. I built the 10' yagi for 88-92MHz.

I'm picking up 50/100 watt high school stations 20, 30 and 40 miles away, along with higher powered stations from over 180 miles away. I haven't even started to do any serious DX work with it.

I'm going to build either the Korner 19.3 for which I've just received the MMANA-GAL files for from Peter Korner or a LFA designed specifically for FM by Free Yagi Antenna Designs for Ham Radio. Justin, (G0KSC) owns InnoVAntenna in addition to the G0SKC website. He and his designs are cutting edge, he uses the latest technology and, how's this for a shock, actually builds his designs.
 
Last edited:
The figures given for the FM log periodic are probably right. It will be large. For a given gain a log periodic will be much larger than a Yagi, but will have greater bandwidth. It is not possible to have a really good indoor FM antenna, as any decent VHF antenna has to have dimensions commensurate with at least half a wavelength: 1.5m.

The J-pole has a big disadvantage for indoor use: it is long (3/4 wavelength) and consists essentially of an end-fed half-wave dipole with a matching section. This makes it very sensitive to surrounding objects, which is exactly the opposite of what you want indoors.

The folded dipole is about the best you can do indoors. It has fairly wide bandwidth, so it is less affected by nearby objects. Note that for good bandwidth don't build it with 300 ohm cable. Instead, cut it out of a roll of aluminium kitchen foil. However, if you do use 300 ohm cable you need to be aware that you can't just shorten it by the velocity factor as this only applies to the internal transmission line mode, not the external dipole mode. If you don't understand what I mean here, then you should simply copy a published design and hope that the designer does understand.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.