The subject is antennas
I'm sure to those that have noticed that I'm having a time keeping up on the blog !!!
The tech net started with me having some mic troubles and i got them resolved. Then Clarence, AE5UB, was wanting some help on choosing a directional array for 6 and 10 meters. I suggested a rotatable dipole for 6 meters. One issue that was brought up is antenna regulations for being in the vicinity of an airport. If anybody has a link to this info please send me a line so I can post it. Multi-band quad was also brought up as an option and there are many plans for multi-band quads on the internet. There was also metion for the log periodic antenna for multi-band use. The Log Periodic is an array that consist of multiple dipoles to cover a wide range of frequency's. The look similar in construction the the old vhf television arrays that everyone had on their house until the mid 90's. As you change frequency the active region shifts along the array.
The windom is a dipole that is commonly used for multi band use. It is an off-center fed dipole. At certain bands and a certain distance that the dipole is fed off center there comes a point that the feed point impedance will be the same . Then the dipole is fed with an impedance transforming balun to bring that high impedance down to 50 ohm.
Tim, KF5FSJ, had an oddity about pouring water at the base of the push-up pole of his 2 meter beam to cure some performance issues he was having.
Dave, NB5B, Is putting up a digipeater in Manford to bring APRS coverage to that area.
Thanks to all that participated and it was a good turnout being that the football game was on.
here is a link to the MP3 of the net to save it right click on the link and click Save Link
http://www.w5bbs.us/?file_id=11
Thursday, October 21, 2010
Monday, October 11, 2010
The Grid Dip Meter
The Grid Dip Meter
The grid dip meter also known as a grid dip oscillator is variable oscillator circuit that uses a variable capacitor for tuning and external coil for the L part of the circuit. Most also have a potentiometer to control the level of oscillation of the circuit. The value of the coil determines the frequency range of the meter and most meters come with a set of coils that are calibrated to the frequency dial on the meter. The actual values of the coils is not critical if you have a receiver with a calibrated readout on it to compare to. The indicator meter on the unit measures the grid current in the circuit and is what is used to make measurements. You can find used meters at hamfest and swap meets in the price ranging from $5 to $50 depending if they have the coils and the quality and condition of the meter
Well the grid dip meter is used to check the resonant frequency of a tuned circuit whether that circuit be a pi-network in an amplifier or a filter or the most common an antenna. How this is accomplished is that rf energy is coupled into the circuit under test and you vary the frequency of the oscillator in the meter until resonance is found. At the point of resonance the circuit will absorb the most rf from the meter much like an antenna will take rf from a transmitter that is matched. Once this happens the grid current of the tube in the oscillator will drop thus the "dip".
If you want to know the value of that inductor or coil you just wound then you can use the meter and a little math to do that. What you would do is take a high quality capacitor of known value and put it in parallel with the unknown inductor and now you have a tuned circuit. Then you just take your meter and find the resonate frequency of that circuit. Now you have a lot of info now what? Well here's the formula to find the inductance of that coil in uH.
.............1
L=--------------------
..........4π2 f2 C
π=3.1416
f is in MHz
C is in uF
L= unknown in uH
The same thing can be done with a known inductor for capacitance and the formula is the same but reverse value C and L in the formula.
You can find the Q of an inductor and here's how to do it:
Find resonance of the industor then tune lower and then higher until the dip is reduced by 30%. When you go higher this is F1 and then lower is F2 and the formula is as follows....
.........F
Q=------------
......F1 - F2
and that Q!!!!!!
The grid dip meter also known as a grid dip oscillator is variable oscillator circuit that uses a variable capacitor for tuning and external coil for the L part of the circuit. Most also have a potentiometer to control the level of oscillation of the circuit. The value of the coil determines the frequency range of the meter and most meters come with a set of coils that are calibrated to the frequency dial on the meter. The actual values of the coils is not critical if you have a receiver with a calibrated readout on it to compare to. The indicator meter on the unit measures the grid current in the circuit and is what is used to make measurements. You can find used meters at hamfest and swap meets in the price ranging from $5 to $50 depending if they have the coils and the quality and condition of the meter
Well the grid dip meter is used to check the resonant frequency of a tuned circuit whether that circuit be a pi-network in an amplifier or a filter or the most common an antenna. How this is accomplished is that rf energy is coupled into the circuit under test and you vary the frequency of the oscillator in the meter until resonance is found. At the point of resonance the circuit will absorb the most rf from the meter much like an antenna will take rf from a transmitter that is matched. Once this happens the grid current of the tube in the oscillator will drop thus the "dip".
If you want to know the value of that inductor or coil you just wound then you can use the meter and a little math to do that. What you would do is take a high quality capacitor of known value and put it in parallel with the unknown inductor and now you have a tuned circuit. Then you just take your meter and find the resonate frequency of that circuit. Now you have a lot of info now what? Well here's the formula to find the inductance of that coil in uH.
.............1
L=--------------------
..........4π2 f2 C
π=3.1416
f is in MHz
C is in uF
L= unknown in uH
The same thing can be done with a known inductor for capacitance and the formula is the same but reverse value C and L in the formula.
You can find the Q of an inductor and here's how to do it:
Find resonance of the industor then tune lower and then higher until the dip is reduced by 30%. When you go higher this is F1 and then lower is F2 and the formula is as follows....
.........F
Q=------------
......F1 - F2
and that Q!!!!!!
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