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Fm antenna matching and SWR.

Posted by svejkovat (My Page) on
Wed, Oct 3, 12 at 1:09

Disclaimer. I live in the country five miles from any neighbor. I'm frankly not worried about potential FCC violations for 1/10 watt FM broadcasting. That said, the following application (i have no access to measuring devices) almost certainly falls far short of even that signal strength.

I went shopping for the cheapest, easiest to use, fm xmitter available so that I could listen, anywhere in the house, yard, basement, or garage, to internet radio broadcasts of which I've recently come to enjoy IMMENSELY. I really liked the idea of dialing into any broadcast I please via any little transistor radio handy while I'm working.

I ordered the following for six dollars with free shipping.
http://www.radio-locator.com/cgi-bin/vacant?select=city&city=49417&state=mi&x=12&y=2
It arrived today. I installed the driver, plugged it in, found a vacant spot on the FM dial and was EXTREMELY impressed with the quality. Good for use in the same room. Unbelievable that this six dollar item has a very useful desktop GUI that provides settings and slidebars for a variety of control over frequency, xmission strength, and other.

So, removing the tiny board from its case I find a roughly three inch antenna lead bunched up along the side. I unfurled this and the broadcast was even better. About 50 ft clear reception.

What I'd like to do is the use the following utility....
http://www.fmuser.com/uploadfile/201004/8/2349199801.png
...to mate this up with a better gain quarter wave ground plane antenna of the type drawn in the gui of that utility.
There are many diy pages on the net that provide the details of construction. My antenna "mast" and ground plane segments will be 3/32 steel welding rod.

I'll connect the usb xmitter directly to the antenna via about two inches of the tiny coax attached to a common SMA connector pigtail (since it's extremely thin and flexible) and then mount the xmitter rigidly to the underside of the "spider"....

http://www.wellshow.com/wp-content/uploads/2010/01/sma-connector-plug-300x270.jpg

...solder the coax to the tiny USB xmitter board at the antenna points and connect the female end of that pigtail to one of these
http://www.wellshow.com/wp-content/uploads/2010/01/sma-pcb-plug-jack-300x170.jpg
...or these..
http://www.wellshow.com/wp-content/uploads/2010/01/sma-4holes-plug-jack-300x170.jpg
... as the body of the antenna "spider" keeping it as lightweight and simple as possible

As a novice, all I seem unable to glean from diy antenna sites and tutorials is SWR matching. Do I need to be concerned with it here?

And lastly, I've seen mention of adjusting the length of the elements after you've calculated them to compensate for different wire materials. Steel, alum, or copper,clad or unclad, exhibit different "skin" effects in these frequencies. Compensation for that can be on the order of an inch or more. Is this correct? What would I calculate for 3/32 steel elements?

If this is beyond the scope of this forum, can anyone please suggest another more appropriate?

Thanks so much.


Follow-Up Postings:

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RE: Fm antenna matching and SWR.

The link you posted for what you purchased is actually the Radio Locator website, but I assume what you purchased looks like a USB thumb-drive but is an FM transmitter.

A quarter wave GP antenna should work and at the power output level of this device, SWR should not be much of an issue as long as you match the antenna and coax. With 50 ohm coax, bending the radials downward to around 45 degrees should match impedance close enough - mount at least 8 foot high off the ground.

The radials can be the same length as the vertical but sometimes they are a little longer - not much of an issue though. Skin-effect differences are also so little that it's not anything to worry about for this application.

You may need to see if an inductor (whether coil or PCB) was used in series with the original antenna to compensate for it's short length and possibly bypass it.

I don't think you're going to get that much power output but be careful - the FCC is completely unforgiving in this area. Google FCC rules part 15 for specifics on FM transmitters such as this.


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RE: Fm antenna matching and SWR.

Oops. Quite right there. Here's the link. Got to be careful anymore. I got "flagged" in another forum for posting these questions since a member was convince that i was trying to sell these transmitters. Even after assurances to the contrary, and reposting my question free of any links to the brand or vendor, he remained unconvinced and kept complaining to the moderators. Wow.

USB2.0 FM Audio transmitter Wireless adapter : eBay

and pics of the inside of device

http://i802.photobucket.com/albums/y...t/SDC14269.jpg

http://i802.photobucket.com/albums/y...t/SDC14278.jpg

http://images15.fotki.com/v588/photo...209/FM2-vi.gif

http://images52.fotki.com/v1551/phot...209/FM1-vi.gif

Since my post I decided to simplify the matter quite a bit by just using a set of rabbit ears, "tuned" to about 53inch total length (both "ears" laid vertically) via an online calculator for 1/2 dipole, and simply soldering the USB card directly to the antenna via 1/4 inch stubs of the flat twin lead emerging from the antenna's base. The little bit of twin lead used to reach the xmitter is negligible in the calcualtions (actually I did shave an approx amount of length from the measurement of the "ears". That was super easy. I rationalized, from the seat-of-the-pants, that thus eliminating any coax or distance between the xmitter and the antenna minimized the necessity for any further balancing via a balun.

I secured this vertically in a first floor hall closet in the middle of the house....
basically its a rabbit ears version of this..
http://www.qsl.net/va3iul/Antenna/Wire Antennas for Ham Radio/16-Vertical Dipole Antenna.jpg

...and ran 12 ft of USB cable down to the laptop in my workshop. It works spectacularly!

Crystal clear stereo fm out to about 150 ft of my house wherein it drops of quite suddenly. I thing the vertical 1/2 wave produces a nice clean toroid of coverage just where I need it without concern for overstepping myself Without a lot of adjustment I seem to have stumbled onto just what I wanted.

The locator site was either out of date or not specific enough to my neighborhood (being based only on zip code).

A couple of the freqs I chose according to that site had rather clear and strong stations already there. I drove around in my car in around an approx 1/2 mile perimeter of my home and located a channel, 106.4 that was dead quiet within 100mz in either direction. Worked out great.


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RE: Fm antenna matching and SWR.

Oops again,

Here's the link...
http://www.ebay.com/itm/USB2-0-FM-Audio-transmitter-Wireless-adapter-/230669438036?pt=Other_MP3_Player_Accessories&hash=item35b4f84c54


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