03-11-2017, 07:51 AM
Wayne,
I can't knowledgeably comment on the first two suggestions but #3 certainly begs a response..
"3. Comet CHA250 vertical. Would also give me a vertical option on the higher bands. Does not need radials. Expensive and the reviews are very mixed especially on its low band efficiency. "
It is not only low band efficiency that is in doubt. The CHA250 uses a large "power swallow" resistor plus a balun effect so that the returned SWR looks ok to the transmitter if the basic feed impedance of the active vertical section varies widely from anything acceptable. The SWR might be ok but the overall efficiency is generally woeful on both transmit and receive. The "Does not need radials." gets me because basically ALL verticals need to operate against a ground plane to get any efficiency, the only exception that readily comes to mind being J-poles.
I think I would probably look at idea #1 more closely. While you don't give any details of the existing mast size, maybe if you can put an insulator under it and base feed it either direct or via selectable loading coils from your remote ATU (you still need ground radials) ? Alternatively maybe use a wire parallel to the mast itself and attach it part-way up, gamma match style, often used by low-banders who use towers with their HF yagis to act as capacitive top hats.. Maybe multiple wires attaching at different heights up the mast until you find the optimum one(s) for the bands you want to use ?
Unfortunately, apart from small rotatable loops with their very narrow bandwidth, antennas generally need to be large and at least 1/4 or 1/2 wavelength (or multiples) to have any efficiency and this, coupled with small yards, just doesn't work out well.
73 Doug
I can't knowledgeably comment on the first two suggestions but #3 certainly begs a response..
"3. Comet CHA250 vertical. Would also give me a vertical option on the higher bands. Does not need radials. Expensive and the reviews are very mixed especially on its low band efficiency. "
It is not only low band efficiency that is in doubt. The CHA250 uses a large "power swallow" resistor plus a balun effect so that the returned SWR looks ok to the transmitter if the basic feed impedance of the active vertical section varies widely from anything acceptable. The SWR might be ok but the overall efficiency is generally woeful on both transmit and receive. The "Does not need radials." gets me because basically ALL verticals need to operate against a ground plane to get any efficiency, the only exception that readily comes to mind being J-poles.
I think I would probably look at idea #1 more closely. While you don't give any details of the existing mast size, maybe if you can put an insulator under it and base feed it either direct or via selectable loading coils from your remote ATU (you still need ground radials) ? Alternatively maybe use a wire parallel to the mast itself and attach it part-way up, gamma match style, often used by low-banders who use towers with their HF yagis to act as capacitive top hats.. Maybe multiple wires attaching at different heights up the mast until you find the optimum one(s) for the bands you want to use ?
Unfortunately, apart from small rotatable loops with their very narrow bandwidth, antennas generally need to be large and at least 1/4 or 1/2 wavelength (or multiples) to have any efficiency and this, coupled with small yards, just doesn't work out well.
73 Doug
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.