Coax
To connect the verticals to the coupler we use 75Ohm coax.
Each vertical has a coaxial line of ¼ wave be aware to use a velocity factor of 82 or 88.
If you use coax with a velocity factor of 66 the ¼ wave lines will be too short to connect them in the middle of the verticals onto the coupler.
How do we make them and measuring them to the correct length of ¼ wave
You can use the same measuring method for the Christman feeding be aware that with a Christman feeding the coax has to be 50Ohm and its not a 1/4 wave that you need.
Like I said before a analyzer is a good investment.
Put on 1 end of the coax a good quality connector and leave the other end of the coax open.
Connect the coax onto the analyzer and cut the coax so you reach the lowest Z on the analyzer.
This was just a quick test to make the screenshot of the analyzer you see it's too high in freq.
You should tune them all the same to around 7,050Mhz.
Calculating them is not that hard
300km/s (speed of light) divide by 4 (1/4 wave) =75
75 divide by freq 7,1Mhz = 10,56 meters
10,56 meters multiply by the velocity factor of the coax 0.88 is 9,30 meters.
If you use 0.82 Vf than the length of the coax will be around 8.66 meters.
If you use 0.66 Vf than the length of the coax will be around 6.97 meters too short to get to the center of the 4sq.
Why do you need to measure the coax with a analyzer if you can calculate them.
Well not all coax are perfectly made and some will be a little of specs of the datasheet provided by the company that made the coax.
Accuracy is good and is still the best start. Don’ think that if you make the ¼ wave coax a little longer the system will not work! It just be a little les perfect.
You see that I said use a good quality connector! That’s for a good reason.
On this side you will find the sentence “ The more a system is complex the more problems there can be. If you use good quality materials you will mostly never have a problem with them.
If you have a problem, searching for the problem can take a lot of time, time that you can’t be on the air searching for the one in a lifetime contact.
15/09/2020
New Coax
I started with PE11 75 Ohm coax but eventually is needed some quality coax
so I can use high power when I buy an other amplifier.
After some search on second hands sites I found some PE14 coax very cheap.
PE11 is good, but the core of the cable can break easily and because I occasionally have to shut down my system (stormy weather), it sometimes happens that I break that core.
I hope that I can avoid this problem with a bit thicker coax.
As soon as I receive the new plugs I will take the opportunity to fine-tune the system a bit.
My email is my call (at) telenet.be
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