cutting antenna, quick question
03-31-2008, 11:43 AM
#21
3rd Gear Member
Join Date: Nov 2006
Location: Dallas, TX area
Posts: 941
RE: cutting antenna, quick question
ORIGINAL: 157db
1/4 waves and half waves are only applicable for transmitting
and getting the antenna to accept a load during transmitting.
I have never heard of setting the SWR for reception.
In fact you have to be transmitting to set the SWR.
Reception does not depend on the length of the antenna after
a certain length.
ORIGINAL: wthalliii
Radio waves have a specific wavelength that depends on the frequency. Antennas work best if their length is an even submultiple of the wavelength. For example, half or a quarter. But in the case of a car radio, it turns out to not be that critical, partially because of the amount of power used by radio stations.
The bottom line: the exact length is not that critical. 14" works a little better if you're in an area that is further from stations, but 8" works almost as well. The nice thing is that you can pretty much make it the length that looks good to you.
ORIGINAL: sevenleaf
That part you lost me.
ORIGINAL: Nuke
It's been decades since I did any antenna design but optimum performance is based on fractions and multiples of the wavelength. For an automobile's FMreceiver, it's not that critical and you'd be hard pressed to notice any degradation in reception performance if you're not dead-on a multiple.
ORIGINAL: sevenleaf
...who cares what size it is really as long as you like the way it looks.
...who cares what size it is really as long as you like the way it looks.
That part you lost me.
The bottom line: the exact length is not that critical. 14" works a little better if you're in an area that is further from stations, but 8" works almost as well. The nice thing is that you can pretty much make it the length that looks good to you.
and getting the antenna to accept a load during transmitting.
I have never heard of setting the SWR for reception.
In fact you have to be transmitting to set the SWR.
Reception does not depend on the length of the antenna after
a certain length.
As a matter of fact, the stock antenna is sized to be 1/4 the wavelength of the FM band. The 14" length quoted by many people here is 1/8 wavelength. If you live in an area that is far from stations, I'd make it 14" or leave it stock. But for many people that live in metropolitan areas, you can pretty much make it any length you want.
Here, don't just take my word for it:
http://www.thechromepony.com/Mods1-04.html
http://mustangworld.com/ourpics/fcar/antenna1.htm
03-31-2008, 02:55 PM
#24
Cut & Paste Expert
Join Date: May 2007
Location: NW Arkansas
Posts: 13,322
RE: cutting antenna, quick question
There are many factors that enter into whether it "matters" whether or not the antenna is a length that is a submultiple of the wavelength. But in some situations, it it does matter for both transmitting and receiving. It is true that there is no standing wave when you're receiving, but that doesn't mean that the length of the antenna doesn't ever matter.
As a matter of fact, the stock antenna is sized to be 1/4 the wavelength of the FM band. The 14" length quoted by many people here is 1/8 wavelength. If you live in an area that is far from stations, I'd make it 14" or leave it stock. But for many people that live in metropolitan areas, you can pretty much make it any length you want.
"At that length, proper commercial products have additional components which "base load" the antenna and make it perform as though it was much longer. There's more than just a short piece of metal involved."
And the 1/8 wave loads into the 50 ohm 1/4 wave tuned coax the same how engineer joe?
The 1/4 wave for FM from the factory matches the 50 ohm coax but god luck getting a cut
down 1/8 wave to load into it the same. Your move.
03-31-2008, 03:31 PM
#25
3rd Gear Member
Join Date: Nov 2006
Location: Dallas, TX area
Posts: 941
RE: cutting antenna, quick question
ORIGINAL: 157db
There are many factors that enter into whether it "matters" whether or not the antenna is a length that is a submultiple of the wavelength. But in some situations, it it does matter for both transmitting and receiving. It is true that there is no standing wave when you're receiving, but that doesn't mean that the length of the antenna doesn't ever matter.
As a matter of fact, the stock antenna is sized to be 1/4 the wavelength of the FM band. The 14" length quoted by many people here is 1/8 wavelength. If you live in an area that is far from stations, I'd make it 14" or leave it stock. But for many people that live in metropolitan areas, you can pretty much make it any length you want.
From your link:
"At that length, proper commercial products have additional components which "base load" the antenna and make it perform as though it was much longer. There's more than just a short piece of metal involved."
And the 1/8 wave loads into the 50 ohm 1/4 wave tuned coax the same how engineer joe?
The 1/4 wave for FM from the factory matches the 50 ohm coax but god luck getting a cut
down 1/8 wave to load into it the same. Your move.
There are many factors that enter into whether it "matters" whether or not the antenna is a length that is a submultiple of the wavelength. But in some situations, it it does matter for both transmitting and receiving. It is true that there is no standing wave when you're receiving, but that doesn't mean that the length of the antenna doesn't ever matter.
As a matter of fact, the stock antenna is sized to be 1/4 the wavelength of the FM band. The 14" length quoted by many people here is 1/8 wavelength. If you live in an area that is far from stations, I'd make it 14" or leave it stock. But for many people that live in metropolitan areas, you can pretty much make it any length you want.
"At that length, proper commercial products have additional components which "base load" the antenna and make it perform as though it was much longer. There's more than just a short piece of metal involved."
And the 1/8 wave loads into the 50 ohm 1/4 wave tuned coax the same how engineer joe?
The 1/4 wave for FM from the factory matches the 50 ohm coax but god luck getting a cut
down 1/8 wave to load into it the same. Your move.
The bottom line is that most any length seems to work pretty well for most people.
03-31-2008, 04:05 PM
#26
6th Gear Member
Join Date: Sep 2006
Location: Gainesville, FL
Posts: 11,278
RE: cutting antenna, quick question
I didn't measure. I just cut it somewhere near the middle. I have a spare atenna anyway, so I wasn't worried. I intially thought I could pull the top off and put it back on, but that didn't work. I just rounded off the top on the bench grinder and primered/painted it.
I had a billet shortie antenna, but it got broken.
I had a billet shortie antenna, but it got broken.
03-31-2008, 05:26 PM
#27
3rd Gear Member
Join Date: Jul 2007
Location: Sterling, Virginia
Posts: 505
RE: cutting antenna, quick question
Holy hell......are we really getting into VSWR and radio wave propogation concerning our car antennae? Chop it till you like it. WE probably play CD's more than the radio anyway. I half expect people to start talking about reflection coeffecients and insertion losses in the next post.
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