Whip length vs frequency

[HogDriver]

I realize physical length of an antenna really only matters when transmitting so my question is, if I make my own whip with a loading coil in the middle, with the bottom section about 6 inches for the 800 band and the top part about 36 inches for vhf hi, will that work pretty well? If I wanted to add a second coil with the middle section about 12 inches for 460 MHz and the top 24 inches, will the combined center and upper work well for vhf hi?

 

[nd5y]

That won't work. It's not that simple.

 

[prcguy]

It would be really complicated making it work. Your lengths are for a half wavelength, which is the first hurdle. An end fed half wave whip is very high impedance, 2,500 ohms or more and you would need a matching circuit for the first 6in section at 800MHz. Then you show 12in for UHF which is another half wavelength and the matching circuit at the bottom of the half wave for the 800MHz side will not work for the UHF side and so on.

If you were working with 1/4 wave whips it would be much easier as a 3" whip matches very well to 50 ohms at 800MHz, then you can make an LC parallel circuit or trap for 800MHz at the end of the 3" whip then add about 2 to 3 inches more to resonant on UHF, then make a trap for UHF then extend the whip above that to resonant at VHF. Its all very basic stuff except your working at very high frequencies and components to make the traps would have to be very small and precise and you would need test equipment to measure the resonant frequency of the traps or the individual whip sections with traps.

I realize physical length of an antenna really only matters when transmitting so my question is, if I make my own whip with a loading coil in the middle, with the bottom section about 6 inches for the 800 band and the top part about 36 inches for vhf hi, will that work pretty well? If I wanted to add a second coil with the middle section about 12 inches for 460 MHz and the top 24 inches, will the combined center and upper work well for vhf hi?

 

[W5lz]

Those coils in antennas such as this are 'stopping/choke' points for lower frequencies. They provide enough inductive reactance to 'stop' higher frequencies, just like you'd cut the rest of the vertical element off. At lower frequencies those coils don't 'stop' things from getting past them so the next section of the antenna plus the lower section is the correct electrical length for the lower frequencies. At llwer frequencies those coils change from a "choke-off" point to a loading coil (adds electrical length). How much electrical length does each coil contribute? I don't know and don't plan on doing the math to find out, you do it!