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1/8 Wave FM Antenna

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All of which goes to show

All of which goes to show that statements about the legality of transmitter power levels *could* be in error if the antenna system used to deliver that power isn't taken into cosideration.

 

 

I would run the EDM transmitter at its lowest power setting of 1 mW and place 48 dB of attenuation between it and the resonant 1/2 wavelength dipole. The field strength will be close to 250 uV/m at 3 meters. Coaxial attenuators can be used and they should add up to 48 dB minus the feedline loss in dB. This cuts things close and your station could still be over the limit. If you want a safety margin you might add an extra 3 or 6 dB. 

Shortening the antenna to 1/4 wavelength reduces the signal by (very) roughly 20 dB and results in a field strength of (very) roughly 5000 uV/m at 3 meters. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

The field strength can be

The field strength can be measured. Amazon sells the RTL-SDR Blog R820T2 receiver and two telescoping dipole antennas for $26. The longer dipole will tune the FM BC band. The receiver reports signal strength in dBm and this can be converted to dBuV/m. 

At 90 MHz a resonant dipole placed in a 250 uV/m field will output -69 dBm to the receiver. If the receiver amplitude accuracy is in doubt it can be calibrated using some precision coaxial attenuators, an RF source, and a power meter or a 200 MHz oscillscope. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

Antenna Factor?

...At 90 MHz a resonant dipole placed in a 250 uV/m field will output -69 dBm to the receiver . ...

A matched, thin-wire, center-fed, 1/2-wave dipole in free space has a feedpoint Z of about 65 ±j0 Ω.*  When used as a receive antenna for a calibrated field intensity meter, the calibration of the receive system must include the Antenna Factor of the receive antenna it uses. 

The graphic below shows the Antenna Factor for a 1/2-wave dipole self-resonant at 90 MHz.  Does the clip above consider this factor when converting power in dBm displayed by a receiver to an arriving e-m field intensity in V/m (or µV/m)?

Also note that the gain and impedance of that dipole can be different for other than free-space conditions, such as when used in an indoor environment.

  *from ANTENNAS, 3rd Edition (Kraus/Marhefka), page 182

*

I used an antenna factor of

I used an antenna factor of 10 dB. Rather than remembering the AF formula one can remember that the AF of a 30 MHz dipole terminated into 50 ohms is 1. The AF at any frequency is linearly proportional to that. At 90 MHz the dipole AF is 90/3 = 3 = 9.5 dB. For fun the calculation was compared to a NEC-2 simulation. 

Today I ordered the $26 SDR receiver and dipoles. I wonder if the FCC FM field agent field strength measurement is per the FCC FM contour method? That method calls for a horizontal dipole 10 meters above ground. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

Using the FRIIS Equation

From reply 34 ... At 90 MHz a resonant dipole placed in a 250 uV/m field will output -69 dBm to the receiver. ...

The classic FRIIS transmission equation shows about -66.1 dBm available at the feedpoint terminals of a 1/2-wave, center-fed, resonant dipole when optimally oriented in a 250 µV/m, free-space field.

The radiated power was 0.001143 mW, although this app doesn't show the complete value.  Tx and rx antenna gains were set to 2.15 dBi, but the app display shows them rounded to 2.2 dBi each.

 

I'm thinking the R&S

I'm thinking the R&S calculator uses a matched load at the receive antenna whereas I used the standard 50 ohm termination used for these measurements. A 50 ohm termination reduces the received power by 1.6 dB compared to 73 ohms. 

 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

I'm thinking the R&S

I'm thinking the R&S calculator uses a matched load at the receive antenna whereas I used the standard 50 ohm termination. Compared to 75 ohms a 50 ohm termination reduces the received power by 1.6 dB. 

 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

FCC Practice

From Reply 36:  I wonder if the FCC FM field agent field strength measurement is per the FCC FM contour method? That method calls for a horizontal dipole 10 meters above ground.

That is doubtful, as the FCC FM broadcast FI curves include temporal and location components, as well as the HAGL of the receive antenna.

I found my error. I had

I found my error. I had remembered the AF rule-of-thumb as being 30 MHz rather than the correct 40 MHz. At 40 MHz (acually 39.5 MHz) the antenna factor of a dipole is 1. So, at 90 MHz the dipole antenna factor is 90/40 = 2.3 = 7.0 dB. The load impedance is 50 ohms.

AF[as dB/m with G as ratio]= 20 Log [9.73)/ (G1/2 λ)]

 http://www.wavetechnology.org/FREEBIES/FCC%20Antenna%20Factors.pdf

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

The $26 USB Dongle SDR

The $26 USB Dongle SDR receiver has arrived and it's very nice! The software loaded easily and it's receiving FM stations. I'm still learning it but it appears to be giving received signal numbers close to FMFOOL.com. I think FMFOOL.com reports signal power into a dipole. 

The plan is to calibrate it for FM field strength measurements. After that I'll see what I can do for AM FS measurements. I have built 1 meter monopole and magnetic loop antennas that will give accurate AM BC measurements. By accurate I mean +/-3 dB. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

How Neat!

"The plan is to calibrate it for FM field strength measurements."

Excellent! We need all the field strength measurement methods we can get!

Carl Blare

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