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- JUNE NEWS LETTER



ISOTRON 20/15/10
17 METERS CAN BE SUBSTITUED FOR ONE OF THE BANDS
SO SIMPLE AND SO RUGGED!

 
This article will also appear on  our web site.

USING A NOISE BRIDGE

There are several analyzers available to help tune an antenna. They have more features than the average operator will use. Most will not measure the antenna values from the radio end of the coax. Some will, but you will have to know the Smith Chart feature of the analyzer. This is normally beyond the scope of the average operator.
 
Who cares? Consider what you want to know when setting up an antenna.
 
With the coax you are going to use connected to the antenna, you will need to know the resonant frequency and the Impedance/Resistance value of the antenna at resonance. The resonant point is the frequency that the inductive reactance and
capacitive reactance are equal. At this frequency you will want 50 ohms of Impedance (Z). This can also be referred to as resistance (R) because the reactance (X) is zero.
 
With the coax connected, the X and Z values of the antenna will not be the same at the radio end of the coax. Different lengths and type of coax will change this value. Therefore, how can you measure the value of the antenna with the coax attached?
 
The RX Noise Bridge is one of the finer instruments to tune any antenna.
 
It simply attaches to the connector of the antenna and your feedline attaches to the Noise bridge. Thereby the instrument is at the antenna where you are working. It also considers the length and type of coax you are using.
 
If the antenna is mounted on a tall pole, simply tilt the pole over and support the antenna end at a convenient height. If it is a telescoping pole, again tilt it over and extend it to the height it will be standing up. Secure the coax to the pole temporarily. Check the tuning with the Noise Bridge. Then re-tract the pole, stand it up and then extend it to where you intended on having it.
 
Ah! You say the Noise bridge requires hearing the receiver and it is down in my basement.
 
This is not a big problem. Using a cordless phone you can attach the base of the phone with an adapter to the phone jack on your radio. You can go to the lower 40, climb a tower or sweat it out in the attic and hear the receiver.
 
Now it is just a matter of following the simple instructions that come with the Noise Bridge.
 
Put the receiver on the frequency nearest to where you will operate. Select SSB mode.
 
There are 2 adjustments on the Noise Bridge, X and R.
 
Adjust both for a deep null in the noise the instrument produces in the receiver. Check the values of the X and R controls.
 
If the X control is on the XC  side, the antenna is high in frequency. If it is on the  side XL side it is low. Make the adjustment on the antenna to zero the X.
 
With the noise nulled out, check the R. You will want 50 ohms ideally. This will give you a 1:1 SWR. So you can see how it works, if the R = 25 ohms, your minimum SWR will be 2:1. It will be the same if R = 100 ohms. Values in between can be calculated as a simple ratio.
 
With a wire antenna that has to be in its normal position to work, drop the feed point so you can access it. Set the Noise Bridge to 0 reactance and the R to 50 ohms. Send the antenna back up into position. Go to your receiver. You will hear the noise generated by the Noise Bridge. Tuning up or down frequency you will find the null. In most cases the null will be deep. If not, then you can bring the bridge down, re-set the R to a different value (such as 75 ohms), send it up and note if the null is deeper or not.
 
Now you have all the information needed from your antenna. No more guessing about your antenna.
 
73,
Ralph WD0EJA
 
01-18
BILAL COMPANY
137 MANCHESTER DR.
FLORISSANT, CO. 80816 U.S.A
PH/FX: 719/687-0650
 
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