trimmerj
Member
Considering building a copper cactus or similar jpole. What sort of bandwidth can I expect in a recieve only enviroment ?
Bandwidth of a JPole
- Thread starter trimmerj
- Start date
- Status
N
N_Jay
Guest
trimmerj said:
It all depends.
What do you consider the "unacceptable" level of degradation to be? :wink:
trimmerj
Member
Probably -6db at 50 mhz away from resonance.
N
N_Jay
Guest
trimmerj said:
And where do you intend resonence to be.
Usually bandwidth is a % of design frequnecy, not a fixed MHz.
Multi element antannas will have other effects as teh various parts interact.
trimmerj
Member
Low band VHF, 39.5 mhz was the target. Wondering how it would do in highband. 158mz would be 4th harmonic
ka3jjz
Wiki Admin Emeritus
John, one thing to keep in mind that as you work away from the designed frequency into harmonics, the radiation pattern (or in this case, the reception) of the antenna tends to change, sometimes dramatically. You might be better off with building a dual-band J pole; such designs exist for 2m/440 ham so it might be possible to do for 39.5/vhf hi (I'm guessing you're getting nosy about MSP hi band usage, right?). I don't have any designs handy at the moment, but a little digging might turn up the correct formulae to use.
73s Mike
P.S. For the doubting Thomases amongst us....Arrow Antennas has been selling dual band J poles (open stub) for some time now. Take a look at their web site
Arrow Antennas
About 2/3 way down the page. This is just an example of what can be done.
73s M.
73s Mike
P.S. For the doubting Thomases amongst us....Arrow Antennas has been selling dual band J poles (open stub) for some time now. Take a look at their web site
Arrow Antennas
About 2/3 way down the page. This is just an example of what can be done.
73s M.
Tweekerbob
Member
- Joined
- May 27, 2004
- Messages
- 614
- Reaction score
- 1
I wouldn't count on -6 db 50MHz wide performance from a low band antena. HFers around the world would bow down and kiss your feet if you could achieve this. This also may be undesirable at the frequency you intend to cut the j-pole for. You wouldn't want high power SW signals intefering with your intended reception band.
As N_Jay stated, it is a % of resonant f, not a fixed and specific bandwidth.
Also, even harmonicas (harmonics) are usually poor performers. You might have better luck with a 3rd harmonic, 158/3=right in the middle of the 6m band. A few scanner antennas rely on this principle.
Ryan
As N_Jay stated, it is a % of resonant f, not a fixed and specific bandwidth.
Also, even harmonicas (harmonics) are usually poor performers. You might have better luck with a 3rd harmonic, 158/3=right in the middle of the 6m band. A few scanner antennas rely on this principle.
Ryan
N
N_Jay
Guest
trimmerj said:
Most circuits resonate on teh odd harmonics not the even.
Al42
Member
Plan on using "tubing" many yards in diameter.trimmerj said:
trimmerj
Member
Was constructed and put on roof yesterday , but made a serious mistake. I used 1/2 '' copper and the damned thing is almost 20' high. I don't expect it to stay up very long.
Hi all,
Sorry, couldn't help the chuckle on this one.
"Was constructed and put on roof yesterday , but made a serious mistake. I used 1/2 '' copper and the damned thing is almost 20' high. I don't expect it to stay up very long."
I wouldn't expect it to either, even unsupported steel will buckle when the first strong wind comes along. You reminded me of a CBer who put up a 100' guyed mast (giant spiders invaded his yard, what a laugh) instead of a tower. That very week one of our famous coastal storms hit, it ended up a tangled mass all over the house, yard, and wherever the bits happened to fall. I couldn't find the antenna, must have been in there someplace!
Diamond makes an excellent discone that is a loaded groundplane on VHF Lo Band and it works very well. Since it's a bit narrow banded down there I suggest borrowing an MFJ antenna analyzer and tuning those elements for the band you want. It may be used for transmitting on 6M, 2M and 70cM so that's why those Lo Band elements are adjustable. The discone part is fixed but extremely wide banded which makes it an excellent scanner antenna good through 800mHz.
Oh, when building a copper cactus don't forget the spines. I was going to make a joke but remembered those static dissipators that look like wire brushes that dissipate the charge on the tower so the chances of a lightning strike are minimized. Hmmm, a lightning proof antenna, maybe some marketing company is interested, maybe Ronco, hmmmmm.
Sorry, couldn't help the chuckle on this one.
"Was constructed and put on roof yesterday , but made a serious mistake. I used 1/2 '' copper and the damned thing is almost 20' high. I don't expect it to stay up very long."
I wouldn't expect it to either, even unsupported steel will buckle when the first strong wind comes along. You reminded me of a CBer who put up a 100' guyed mast (giant spiders invaded his yard, what a laugh) instead of a tower. That very week one of our famous coastal storms hit, it ended up a tangled mass all over the house, yard, and wherever the bits happened to fall. I couldn't find the antenna, must have been in there someplace!
Diamond makes an excellent discone that is a loaded groundplane on VHF Lo Band and it works very well. Since it's a bit narrow banded down there I suggest borrowing an MFJ antenna analyzer and tuning those elements for the band you want. It may be used for transmitting on 6M, 2M and 70cM so that's why those Lo Band elements are adjustable. The discone part is fixed but extremely wide banded which makes it an excellent scanner antenna good through 800mHz.
Oh, when building a copper cactus don't forget the spines. I was going to make a joke but remembered those static dissipators that look like wire brushes that dissipate the charge on the tower so the chances of a lightning strike are minimized. Hmmm, a lightning proof antenna, maybe some marketing company is interested, maybe Ronco, hmmmmm.
- Status
Similar threads
