Multi-Band Scanner Antenna Question
- Thread starter CTorris
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Techy
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CT, no that would not need a balun at all. That will be 50 ohms using the SO-239 connector.
kf5bti
Member
No balun needed, its direct fed using Coax. I used those plans to build my scanner antenna in my attic but did slightly modify the lengths to better meet my listening needs. I mostly listen to a 800 mhz public safety trunking system, but also hear all of the surrounding county VHF/UHF systems and can hear air band coms quite well (Located about 12 miles from the airport.)
Ok i just built the above multi-band antenna..... Any ideas on how to make a mount for it? I also made a off center dipole 48" x 18" (1/2" copper tubing) and was woundering how close can i mount both of them together? Thank you all for your help!
http://www.qsl.net/n4yek/scanner/antenna.pdf
http://www.qsl.net/n4yek/scanner/antenna.pdf
kf5bti
Member
Where are you going to mount them? Mine are mounded inside my attic and are suspended using string from the rafters above. As far as spacing try to keep them 1/4 wavelength apart and away from any metallic items such as HVAC ducts. (1/4 wavelength for these frequencies is roughly 19")
LIScanner101
Completely Banned for the Greater Good
Speaking of baluns...
My antenna came with one. It looks like the 300 ohm-to-75 ohm balun you’d see attached to a TV antenna so you can use RG-6/U coax instead of 300 ohm twinlead. My question is, how critical is it to use this device on a RECEIVE-ONLY antenna? Can’t I just attach the coax cable braid to the lower dipole directly and the center conductor to the upper? I guess that’s taboo but I don’t understand why. BTW, I’m considering trashing the old RG-6/U coax that came with the antenna and getting some RG-11/U (which is still 75 ohm but thicker and less "lossy") but I'm not understanding how a balun works in the whole scheme of RECEIVE-ONLY antennas....THANKS!!!
My antenna came with one. It looks like the 300 ohm-to-75 ohm balun you’d see attached to a TV antenna so you can use RG-6/U coax instead of 300 ohm twinlead. My question is, how critical is it to use this device on a RECEIVE-ONLY antenna? Can’t I just attach the coax cable braid to the lower dipole directly and the center conductor to the upper? I guess that’s taboo but I don’t understand why. BTW, I’m considering trashing the old RG-6/U coax that came with the antenna and getting some RG-11/U (which is still 75 ohm but thicker and less "lossy") but I'm not understanding how a balun works in the whole scheme of RECEIVE-ONLY antennas....THANKS!!!
Baluns
From what i understand on a off center dipole antennas your trying to receive two separate signals. Like a tv a tv antenna vhf-uhf-fm signals. You can only watch one channel. A balun separates the signals so there not mixed. The way your hooking it up, center of cable to one pole and branded outer shield to the other pole is combining the signals. A balun is required for all dipoles. I hope this helps
From what i understand on a off center dipole antennas your trying to receive two separate signals. Like a tv a tv antenna vhf-uhf-fm signals. You can only watch one channel. A balun separates the signals so there not mixed. The way your hooking it up, center of cable to one pole and branded outer shield to the other pole is combining the signals. A balun is required for all dipoles. I hope this helps
nd5y
Member
Baluns aren't critical for something like a scanner antenna. Directly attaching the coax will work.
The primary purpose of a -balun- is to change from a balanced state to an unbalanced state. It has a secondary purpose of changing impedance in fairly large 'steps'.
As was said, for a typical 'listening' antenna a balun isn't absolutely necessary. Not using one typically means a slight 'shift' or change/skewing in shape of the reception/transmitted radiation pattern. That 'shift' isn't much noticeable with omnidirectional antennas, a lot of other things can produce that same 'shift'.
Baluns do work well for their intended purpose(s), but they are not a 'cure all' by any means. If one isn't needed then don't bother.
- 'Doc
As was said, for a typical 'listening' antenna a balun isn't absolutely necessary. Not using one typically means a slight 'shift' or change/skewing in shape of the reception/transmitted radiation pattern. That 'shift' isn't much noticeable with omnidirectional antennas, a lot of other things can produce that same 'shift'.
Baluns do work well for their intended purpose(s), but they are not a 'cure all' by any means. If one isn't needed then don't bother.
- 'Doc
n4yek
Member
True but since that feed point is about 300 ohms impedance it won't hurt to go to Walmart and pick up one for a few bucks: GE Transformer for F and Spade Lug Connection: TV & Video : Walmart.com
or at Radio Shack for $6 Indoor/Outdoor Matching Transformer : Antenna Transformers | RadioShack.com
That antenna is great, I have used it for about 5 years. I used 75 ohm RG-6 coax to connect it to my scanner.
kf5bti
Member
A ground plane antenna such as the one disucussed earlier in this thread is not a 300 ohm impedance. The feed point would be somewhere around 70 ohms for a ground plane antenna.
Oh, and to complicate it a bit more...
A typical 1/4 wave groundplane antenna has an input impedance somewhere between about 25 ohms and 75 ohms. It will depend on how much 'droop' there are in the radials (one way of adjusting them). If those radials are straight horizontal then figure an input around 20 - 30 ohms. As you start to make them 'droop' the impedance starts going up until when they are straight down (vertical dipole) the impedance averages around 70 - 75 ohms.
The particular multi-band groundplane antenna from the link will have an impedance of around 50 ohms generally. If it's fed with 50 ohm coax it really wouldn't require a balun or any special impedance matching. If you feel you have to use a balun then use a 1:1 balun. Or just wind a choke and use that.
- 'Doc
A typical 1/4 wave groundplane antenna has an input impedance somewhere between about 25 ohms and 75 ohms. It will depend on how much 'droop' there are in the radials (one way of adjusting them). If those radials are straight horizontal then figure an input around 20 - 30 ohms. As you start to make them 'droop' the impedance starts going up until when they are straight down (vertical dipole) the impedance averages around 70 - 75 ohms.
The particular multi-band groundplane antenna from the link will have an impedance of around 50 ohms generally. If it's fed with 50 ohm coax it really wouldn't require a balun or any special impedance matching. If you feel you have to use a balun then use a 1:1 balun. Or just wind a choke and use that.
- 'Doc
Note that the balun often recommended when using the OCFD is really a balun AND a transformer, in this case a 4:1 ratio, ie what is usually seen as a "75 to 300 ohm" balun/transformer.
The OCFD needs a transformer, as the element lengths were carefully chosen to present a reasonable range of impedances, from about 100 to 400 ohms as you sweep across the bands, and the transformer presents a better impedance match to the 75 ohm cable.
The elevation angles at uhf and above looking nearly straight up are another story covered elsewhere.
The "balun" part is basically non-existent electrically due to the severe physical/electrical imbalance of the antenna itself swamping any balun action, and the fact that most tv-type balun/transformers are voltage types and not current type. So all that is really active is the transformer, and a heavy common-mode interaction with the cable braid since there is no balancing taking place.
One suggestion when using a tv-type balun/transformer for an OCFD is to use one that has separate isolated windings. Some have shared grounds, and this only aggravates the common mode problem. The isolated windings help a little bit here. Ferrites can also be used externally but in the environment for an ocfd, one usually doesn't spend a lot of time trying to perfect it.
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If I remember right a 1/4 wave monopole over perfectly conducting infinite flat ground has an impedance around 35ohms and the match can be pulled around a bit by slightly shortening or lengthening the whip. Going from an infinite ground to 3 or 4 radials would raise the impedance some and drooping them also raises the impedance to a max of about 72ohms in free space when the radials are parallel but opposite the whip as in a coaxial dipole.
Bottom line is you can usually adjust the whip length to get a perfect match as long as there is a reasonable ground plane under the whip. A whip slightly shorter than resonance will raise the impedance and longer will lower it to help match to various ground plane conditions.
prcguy
Bottom line is you can usually adjust the whip length to get a perfect match as long as there is a reasonable ground plane under the whip. A whip slightly shorter than resonance will raise the impedance and longer will lower it to help match to various ground plane conditions.
prcguy
n4yek
Member
True but post #4 isn't a ground plane antenna
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