Ham Radio Forum

i have fabricated my own 6 element Yagi Uda for a half wavelength (as my transmitter) at about 99MHz, and used a balun i took out of a VHF antenna to hook it up to the antenna. But the weird thing is that when i do this, the signal which is clearly heard on my receiver from near without any antenna is lost. My attempts to move the receiver which i soon hook up with a monopole antenna receives nothing from the Yagi-Uda. Is this a problem with the impedance matching? My calculations on my Yagi-Uda shows an impedance of about 60 ohm while I am using a coax cable of 75 ohm to connect the balun to my transmitter. How do i rectify this problem?

The result you are having is not due to the slight impedance mismatch between the 60-ohm antenna and the 75-ohm feed-line. To the contrary, it more likely is happening because you have a relatively good impedance match.

If I understand the situation correctly you are wondering why you no longer hear your signal on a receiver located near the transmitter when the transmitter is connected via a feed-line to the Yagi-Uda antenna. When you transmit without the antenna connected, your transmitter radiates a significant amount of signal locally into the nearby receiver, but when you connect the antenna via the feed-line nearly all the transmitter power goes up the feed-line to the transmitting antenna and is radiated there from much farther away. That causes the signal near the transmitter to be much weaker, which is normal. I assume you are using a very low power transmitter. If that is true, it is not surprising that the RF signal strength near the transmitter is much weaker or even not detectable with the feed-line and antenna connected.

Most people probably would be surprised to learn that it often is difficult to receive a decent TV signal from a high-power television station with a TV receiver located inside the transmitter building. There usually is an antenna radiation null directly below a TV transmitting antenna which makes the signal from the antenna weak. The transmitter is shielded and nearly all the RF power it generates goes up the feed-line to the antenna. The result inside the building is a weak signal from the antenna and a weak signal from the transmitter which combined to produce a snowy picture with a strong ghost image. It usually is necessary to mount a receiving antenna outside where it can receive more signal from the transmitting antenna to obtain a decent off-air picture.

ADDED NOTE
I just noticed the part where you wrote "My attempts to move the receiver which i soon hook up with a monopole antenna receives nothing from the yagi-uda." If by that you mean that you receive nothing when the receiver is moved near the transmitting antenna, then something obviously is wrong. However, the slight impedance mismatch could not be the problem. I would look for a shorted or open feed-line.

-Bob

I finally got the antenna working correct. I looked over the antenna and discovered a shorted wire on my balun and fixed it. The antenna was not accurately aligned but it still gives quite a good gain. The radiation on the sides are much more weaker now with the antenna fed properly.

Thanks for the advice.

Is there a way for me to measure my transmitter's power? I don't have those hi-tech meters ( i forgot the names) that i keep stumble about through the internet, is there any alternative way to do it?

It is possible to accurately measure transmitter output power without a power meter, but it is necessary to have a calibrated measuring instrument of some kind. For example:

1) RF dummy load temperature rise can be measured and that measurement can be converted to power if the dummy load doesn't produce any light and all the heat produced by the dummy load can be trapped and accounted for.
2) The method above is generally difficult outside a laboratory environment. However, another variation of that method is to measure the temperature of an RF dummy load with transmitter power applied, let it cool, then connect the same dummy load to variable low-frequency AC or DC power source, adjust the magnitude of that power source to match the temperature rise when the dummy load was connected to the transmitter, measure the dummy load voltage and current at temperature, and multiply them together (P = E * I) to calculate the power being dissipated in the dummy load.
3) If you have access to a calibrated oscilloscope you can use it to measure the peak-to-peak RF voltage across a dummy load of known resistance connected to the transmitter, convert the peak-to-peak measurement to RMS voltage (RMS = 0.5 * .707 peak-to-peak voltage), and then calculate the power using P = E^2 / R.

Other methods can be used, but they all require access to one or more calibrated measuring instruments of some kind.

-Bob