The Survival Podcast Forum

Survivalism & Self Sufficiency Topics => The HAM Radio Board => Amateur Radio How-To's => Topic started by: Greekman on April 24, 2015, 02:04:05 AM

Title: Ballancing the antenna with a length of coax?
Post by: Greekman on April 24, 2015, 02:04:05 AM
in this construction
http://f5ad.free.fr/Liens_coupes_ANT/G/KC4FWC%20Folded%20GPA%20400.htm
the guy is balancing his folded monopole with  length of coax

Quote
the folded monopole when constructed as such presents a 100 ohm impedance. In order to match 100 ohms to 50, we can use an odd quarter wavelength of 75 ohm coax as listed in the picture. To figure this, take the impedance given times the desired impedance (100 x 50) = 1500 and take the square root of that number (which is 70.7) and that is the feedline matching section you are looking for. In this case, we use 75 ohm since it is so close. Next, to find the length, you must know the coax velocity factor. Foam cables are usually 88% and solid dielectric cable is around 66%. Let's use foam cable here. Now, find the frequency you are looking for which is 445 MHz. Take 2808 and divide by 445 and get 6.3 inches. Now, times that by the velocity factor of 88% (0.88) and get approximately 5.5 inches. This is ONE ELECTRICAL QUARTER WAVE LENGTH FOR 445 MHz using cable with an 88% velocity factor. You may use this length for your matching section, or times it by 3 and get 16.5, or times it by 7, 9, 11, or whatever you desire so long as it is an ODD multiple. Then, stick a barrell on the other end and run it to your 50 ohm transmission line

my question is: Can this trick be used in any case we have an non-50Ohm antenna?
Title: Re: Ballancing the antenna with a length of coax?
Post by: Carl on April 24, 2015, 07:05:14 AM
in this construction
http://f5ad.free.fr/Liens_coupes_ANT/G/KC4FWC%20Folded%20GPA%20400.htm
the guy is balancing his folded monopole with  length of coax

my question is: Can this trick be used in any case we have an non-50Ohm antenna?

Sometimes...but it is better to use a shunt match or gamma match and leave the cable and equations alone .There is nothing like a properly designed antenna (shunt matching is used most often,like the match on the BIG STICK antenna)