Today I had struggle with two antennas, where I have too small experience about. Probably someone may give an advice.
One was car radio of 8 Watt 27 MHz. As the vertical size is hardly limited, I got a POM plastic round rod of 8mm diameter, wound the 70 mm zone of 0,50 mm wire turn to turn and then 150 mm zone of winded with step of 4 mm. Totally antenna had 220 mm length (so short). Then I measured the SWR of 1,2...1,25 what is rather good, but its not detrimental to want even better, therefore I tried to change all those sectors slightly, 5 mm longer or shorter. All cases antenna got SWR of 1,6-1,8 so the sizing is very critical and cannot be changed no a yotta. How then to get a better SWR?? Changing the diameter?
Other was marine radio of 30W, 150-170 MHz. There I construct a 12mm copper waterpipe in the base and 10 mm in upper part, kept together by porcelain tubule with small gap between pipes in what the coaxial is soldered, one wire to one pipe, and braid to another. Yet SWR was 2,5 what is unexpected bad. Yet total length is 90 cm thus the each pipe is exactly quarter wave. How can be so drastically bad result?? Probably its a rare defect in coaxial cable (it was used beforehand for who knows what)? If I count 73/50 Ohm its about SWR=1,2 in worst case. What I should change here? The gap probably??? Or different copper pipe diameters may cause such problem? Or problem is caused due the coaxial is laid by inside the lower pipe, so pipe end, where the voltage component gets maximum, is physically very near to braid, where unbalansed voltage is nil. Or the very basics is that radio output is unbalanced but antenna is symmetrical, so the balun is must to be?
One was car radio of 8 Watt 27 MHz. As the vertical size is hardly limited, I got a POM plastic round rod of 8mm diameter, wound the 70 mm zone of 0,50 mm wire turn to turn and then 150 mm zone of winded with step of 4 mm. Totally antenna had 220 mm length (so short). Then I measured the SWR of 1,2...1,25 what is rather good, but its not detrimental to want even better, therefore I tried to change all those sectors slightly, 5 mm longer or shorter. All cases antenna got SWR of 1,6-1,8 so the sizing is very critical and cannot be changed no a yotta. How then to get a better SWR?? Changing the diameter?
Other was marine radio of 30W, 150-170 MHz. There I construct a 12mm copper waterpipe in the base and 10 mm in upper part, kept together by porcelain tubule with small gap between pipes in what the coaxial is soldered, one wire to one pipe, and braid to another. Yet SWR was 2,5 what is unexpected bad. Yet total length is 90 cm thus the each pipe is exactly quarter wave. How can be so drastically bad result?? Probably its a rare defect in coaxial cable (it was used beforehand for who knows what)? If I count 73/50 Ohm its about SWR=1,2 in worst case. What I should change here? The gap probably??? Or different copper pipe diameters may cause such problem? Or problem is caused due the coaxial is laid by inside the lower pipe, so pipe end, where the voltage component gets maximum, is physically very near to braid, where unbalansed voltage is nil. Or the very basics is that radio output is unbalanced but antenna is symmetrical, so the balun is must to be?
