Antenna Length, RF Band and Impedance Matching

Our antenna band appeared to be shifted to 71 or 76 MHz.

Preeti and Roopa were looking up reasons for which the antenna might not be working at the desired frequency. They came across this fact on this page.

Antenna length

The length of a dipole is the main determining factor for the operating frequency of the dipole antenna. Although the antenna may be an electrical half wavelength, or multiple of half wavelengths, it is not exactly the same length as the wavelength for a signal travelling in free space. There are a number of reasons for this and it means that an antenna will be slightly shorter than the length calculated for a wave travelling in free space.

Hence, we feel that we have actually used a longer length than necessary...giving us a smaller frequency at 72 MHz. We will try reducing the length tomorrow and checking.

Also regarding the impedance matching..a Sleeve Balun is commonly used in VHF range. You can find more details on this page .

Bandwidth and antenna windings

Our dipole antenna element was fixed on wooden frame using nails. The dipole wire was wound around a nail to make it taut. When we introduced a wound around the nail, they acted as an inductors on ends of the wire (Rad's comment with a chuckle).

We need to put plastic caps on each end, and then use some material to fix the dipole wire, possibly some rope or hard plastic cover on usual copper wire. see this link.




As for the interference outside the "allowed band" of the dipole, let us wait for Onkar to fetch the co-axial cable from NCRA/TIFR, Pune.

Antenna and the bandwidth

We put up our first dipole antenna element on Friday (?) afternoon. Here is Roopa trying to keep the antenna wire taut, and Onkar's pic does not need a caption ;-)

The picture on the right shows the band. Even though Onkar tried to measure the length of the dipole as LAMBDA/4 on each side, the band is centered on 71.5 MHz. What gives? The bandwidth is barely 1 MHz, kind of a let down. At the same time, there is a lot of interference from the surroundings. Perhaps our antenna wire (not a co-axial yet) is too prone to pickup from around.

AD633 for multiplier

Vamsi suggests using Analog Devices' AD633 as the multiplier in our project. The circuit consists of a standard implementation mentioned in the datasheet. (Fig. 3).



Specifications

Transfer Function		[(X1–X2)(Y1–Y2)/10] + Z
Slew Rate (V/µs		20V/µs
Supply Voltage (V max)		±18V
Supply Current (max)		6mA
Temp Range		-40 to 85°C
Package		DIP/SOIC

From Analog Devices website. The datasheet.

AD633 seems cheap, and very popular. Good for it. Also, AD seem very forthcoming to give free samples. I will try something about that.

The development board for PIC18F2455

The previous post contains the link to the datasheet of the PIC18F2455. That thing is huge, some 428 pages. Instead, here is the link to the development board of the same controller. With the development board, I hope, it will be easier to handle USB protocols. (Without it, it seems simply impossible). I havn't been able to fix myself on either of the modules till now. The options are:
1)the dlpdesign module that mandar discussed
2)this PIC module
3)Since I am comfortable with the 8051 architecture, I would like to use a 8051 based controller, am trying to find a suitable one. Any help will be appreciated :-)

Here is the link:
http://www.sparkfun.com/commerce/product_info.php?products_id=762

Using a USB PIC

The USB microcontroller I have in mind is the PIC 18F2455.

Here is the link for the datasheet. ww1.microchip.com/downloads/en/DeviceDoc/39632b.pdf

This microcontroller not only offers upto 8MHz internal clock, but also has features like setting a self sampling rate through software, an inbuilt ADC (not sure if we can use it though). It provides for having 2 different rates for running the controller and for USB data transfer!

Dipole Antenna

Hi guys,
I have got 1kg (~50m) of aluminium wire (gauge 12) for making the dipole antenna. I got a good deal for Rs. 230/-. Copper wire is pretty expensive and 16m will cost Rs. 800/-. We can buy the copper wire whenever needed. We would be making a wooden stand in the workshop soon.