An FM Transmitting Antenna

Part 15, Low Power, FM Stereo Transmitter on a Budget - Part Two...

After the success of the FM Stereo Broadcaster, I decided I'd like to have a proper antenna for it - rather than the usual 'piece of wire' hanging out the back.

Consider the options:

  • A collapsable whip antenna - Radio Shack offers a few. Simple, cheap, but not what I'm looking for.
  • A Ground Plane - nice, but the ground-plane takes up a lot of space. Arrow Antennas will cut one for a specific FM Broadcast frequency (GP Custom model).
  • A Yagi. Great if I'm trying to send in one particular direction OR increase the effective radiated power in that direction - a no-no, given FCC rules for Part 15.
  • Ramsey Electronics offers a TM100-Tru-Match FM Broadcast Antenna Kit for only... $69.95! I like that one - it's a real 3-meter, folded-dipole and is mostly 'flat', except for the PVC housing.

Taking Stock

I like the appearance of the TM100 and it doesn't take up the space of a ground-plane. So our new antenna will be the TM100 - but wait: the cost ($70.00 + shipping) is a bit high for what's in the box. If you live where you don't have DIY resources this will have to do; but, perhaps we can do better.

The assembly manual for the TM100 is provided online (PDF File). A close examination shows what you get for your $70 (plus shipping):

  1. A length of 300 ohm twin-lead, typical: Radio Shack 15-1175 (100 feet), about $18.69.
  2. A 75-to-300 ohm balun (transformer).
  3. A 3' length of coax.
  4. A Ferrite choke core - ostensibly to keep any transmitted RF from reflecting back to the transmitter on the outside of the coax.
  5. An "F" Connector - specifically, a panel mount "F" connector to feed through the PVC enclosure
  6. Some bits of PVC to provide the "fancy" enclosure. Eh, call it a Radome if you like.

The Plan

Most of the parts are things I already have on-hand. But let's do a cost analysis if we had to buy everything.

Various Antenna Bits - VHF Balun and Twinlead Folded Dipole Various Antenna Bits - I had several VHF baluns in my cables, cords and wall-wart box, but I bought another Balun for about US$2.75.

Twin-Lead: I have a factory-supplied, 300-ohm twin-lead, probably provided with some FM receiver I bought years ago. Estimated $3.50 if you had to buy one. It measures 57 inches and that should work and will allow me to trim for the middle of the FM Broadcast band.

Balun for 75-to-300 ohms: Ditto - from my big box of antenna / connector / cable / wall-wart bits in the garage. Free to me, but I actually priced one at Fry's for $2.75 (see above).

Coax: - Ramsey specifies RG-174U (which is pretty lossy) and is 50 ohms - an odd choice. It can't be much of a "Tru-Match" if you feed a 300 ohm antenna through a 75-to-300 ohm transformer, with a 50 ohm coax. I actually tested this using RG-174 and had quite a bit of coax radiation, so I opted instead for 75 ohm RG-59U. Again, free to me, but probably around $3.00 and you'll have several feet left over.

Ferrite Choke Core: I've got the on-hand, but they're also readily available. Parts onhand, but I used a snap-on version just like this one from All Electronics $1.25

A trip to Lowes provided the 1" PVC bits (about $5), and Altex supplied a panel-mount "F" connector $0.95). Not listed is a ring-mount, solder-tab to fit the "F" connector: we'll have to solder the coax shield to something, since PVC is non-conductive. I think it was $0.10.

300-to-75 ohm Balun Crack open one of the 'Quick-Connect "F" Plug and Transformer's and you'll find a little 300-to-75 ohm VHF balun.

The tiny transformer can be clipped out of the plastic case (above) and used inside the FM-Antenna's PVC enclosure. Solder the TwinLead Antenna to the left-hand side of the above transformer. Then note which pin goes to center and shield of the coax-side and solder to the 75-ohm coax.

So, with a little PVC glue and some fender washers thrown in the TM100 Tru-Match FM Broadcast antenna can be cloned for about $16.55. My Cost is quite a bit less since I had most of the parts on hand: about $6.05 for 10-feet of 1" PVC, three caps, one "T", one "Elbow", a panel-mount "F" connector and a solder-tab. Ramsey provides the PVC tubing in short pieces (10" or 12.625" pieces), so we'll come out a bit ahead not having to use couplers to join small pieces and the final result will be just a bit tidier. I'm guessing the reason for this is to get the product small enough for shipping. Not bad — and as Tom General (KE5ICX) says "...for the cost, the darn thing will be rather impressive to look at."

So, since anything worth doing is worth over-doing — Let's clone that antenna!

The Math

The antenna is a full-wave (3 meters) dipole, but folded in half. So, the principal formula used here is the formula for the length (L) in feet, of a half-wave dipole when given the desired frequency (F) in Mhz.:

Starting with this Formula:
wavelength (in meters) = 300 / frequency in MHz

This formula will give us the length of the antenna, end-to-end (as folded), for a given frequency. Since a Folded Dipole has a relatively good bandwidth we'd really just like to put the tuned length of the antenna in the center of the FM band, since the FM Stereo Broadcaster will probably not be set to a single frequency, will probably move to different frequencies to avoid nearby strong broadcast stations or localized noise.

Let's calculate a typical folded-dipole, made of twin-lead, centered on the FM Broadcast band at 98.0 Mhz. Our twin-lead should have a velocity factor (the speed at which radio waves travel down the wire) which will correct our 'calculated' length for 'real-world' length:

  • Velocity Factor of typical Radio Shack Twin-Lead is .82 (or 82% of the speed of light)
  • A Full Wavelength (in meters) = 300 / frequency in MHz
  • 1/2 Wavelength (in meters) = (.82) * 300 / 98.0 MHz / 2 = 1.255 meters
  • 1 meter = 39.3700787 inches, so 1.255 m = 49.41 inches

Calculated Half-Wave Dipole Lengths - FM Broadcast Band
FM Frequency (Mhz)88 Mhz98.0 Mhz108 Mhz
Calculated Length (inches)55.0"49.4"44.8"

But experimental measurements (at least for my materials and configuration) indicated that possibly, my OEM Twin-Lead was a bit too long. I started with the OEM length, 57" (or 28.5" each side) and this seemed to peak at around 90.3 Mhz.

This could be due to additional capacitance in the setup or some other vagary in the makeup of the materials. Your mileage may vary. I shortened the antenna by about 1.5", or about 3/4" at each end, making the new antenna 27.75" on each side or 55.5". I found the new length appears to peak at around 101 Mhz.

Results

FM Stereo Broadcaster - Front Panel

Finished Product is Identical to the Ramsey TM100 - a Clone!

Epologue

Any mistakes are probably my own. This project is not guaranteed for any specific outcome or purpose.

Warning! If you intend to mount the antenna outside, please pay attention and do not locate the antenna near overhead power lines or other electric light or powered circuits, or where it can come into contact with any high voltage.

Also: Anything mounted outside, on or above a roof, should have a lightning protection system, such as an in-line spark-gap and a coax shield ground, bonded to house electrical ground (as per National Electrical Code). NEC Article 810 (A) thru (K) covers radio transmitting and receiving equipment and is intended to prevent (or reduce) voltage surges caused by static discharge or nearby lightning strikes from reaching the center conductor of the coax lead-in.

I cannot be held responsible for illegal uses. This project is solely for educational, hobbyist, and experimental purposes. Please check your local (and national) regulations regarding unlicensed transmissions. FCC's Part 15 Regulations are recommended reading, particularly Bulletin 63 (October 1993) "Understanding the FCC Part 15 Regulations for Low Power, Non-Licensed Transmitters". This project describes an "Intentional Radiator" and as such (operating in the band 88 - 108 Mhz), according to Section 15.239 (b): "The field strength of any emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters." Using their calculations, this works out to about P = 0.3 E2 watts, or 0.3 x (250 x 10-6V)2, or 0.3 x 0.0002502, or: 0.00000001875 watt (.01875 μ-watt).

Epologue

  • 3 August 2010 — Updated 'typical' Twin-Lead Calculations, including a Velocity Factor for Twin-Lead.