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-   -   Any FM antenna for a truck? (http://forum.tvfool.com/showthread.php?t=6828)

sc2dave 8-Jan-2012 12:27 AM

Any FM antenna for a truck?
 
Is there anything that i can make that's better than the stock antenna? I have an 87 Toyota truck,if that helps.

GroundUrMast 8-Jan-2012 1:41 AM

For FM reception in motion, the stock 'whip' antenna on most vehicles is at or near resonance and is a very good antenna for the application. In theory you could look at 5/8th wave and collinear antennas. Both of these options have the potential to lower the angle of radiation closer to the horizon which would give you a few dB of gain compared to the 1/4 wave over ground plane (the typical stock automotive FM antenna concept).

Search Google, Yahoo, Bing, etc.

sc2dave 8-Jan-2012 5:38 AM

ok. So how do i determine the length for a certain station?

GroundUrMast 8-Jan-2012 3:56 PM

https://www.google.com/search?hl=en&...1477l0.6.2l8l0

https://www.google.com/search?hl=en&...l536l0.2.1l3l0

sc2dave 8-Jan-2012 4:29 PM

what do i input for Mhz? 100 would be the middle of FM Radio,right? I did that and got 5.8500000000000005 feet.Is this right?

GroundUrMast 8-Jan-2012 5:15 PM

100 MHz is right... or certainly close enough.

A vertical antenna with gain is going to be larger than a 1/4 wave whip mounted on a fender.

Then you'll need to consider impedance matching...

A successful antenna design & build project is more than cutting a piece of tubing and soldering a connector on. If you have the knowledge, tools and test equipment, you've still got some work ahead of you.

sc2dave 8-Jan-2012 7:24 PM

Quote:

Originally Posted by GroundUrMast (Post 19069)
If you have the knowledge, tools and test equipment, you've still got some work ahead of you.

That's just it, I don't know anything about antennas :( .I'm like a 5 year old learning how to fly an aircraft!

sc2dave 11-Jan-2012 2:45 AM

so a 5 foot antenna? How much better will reception get?

GroundUrMast 11-Jan-2012 3:12 AM

Quote:

Originally Posted by sc2dave (Post 19175)
so a 5 foot antenna? How much better will reception get?

With a well tuned, custom built collinear, you would be doing quite well if you achieved 6 dB gain compared to the stock antenna. That would theoretically let you receive the same signal strength at double the distance.

I sincerely wish I could share over four decades of learning and real world experience in just a few posts. I'm simply not that effective a writer.

Take a look at this source of information: http://www.radio-electronics.com/inf...nnas/index.php

and

http://www.dxzone.com/tag-fm-antennas/ Not all of it is FM broadcast band related but look for 'home-brew' and 'matching'.

sc2dave 17-Jan-2012 2:05 AM

what about an antenna resting on the roof of my truck? Fractal antenna any good?

GroundUrMast 17-Jan-2012 5:47 AM

For a vehicle in motion or frequently moved then parked in various locations and facing a random direction, you'll need an antenna that has fairly equal reception from all directions.

As I mentioned in post #2 of this thread, 'gain omnis' such as collinear and 5/8 wave verticals achieve gain by focusing on the horizon more than the shorter and simpler 1/4 wave over ground plane vertical antenna. Both the collinear and 5/8 wave designs incorporate some type of impedance matching network so they can efficiently couple received energy into the feed line. Simply making the antenna element longer will detune the antenna leaving you with less signal to the radio.

Most of the fractal antenna designs I've seen tend to be somewhat directional. Those designs would provide gain in one or two directions and less gain in the other directions. However, if you can provide a link to one or more fractal designs, I'd be happy to look them over for you.

sc2dave 18-Jan-2012 2:17 AM

1 Attachment(s)
Here it is.

GroundUrMast 18-Jan-2012 2:57 AM

If I can find time in the next few days, I'll model the drawing in 4NEC2. I'll have to estimate some dimensions since they're not provided.

Without modeling anything though, this looks like a UHF or cellphone band bidirectional antenna intended to operate somewhere in the range of 470 to 1200 MHz. This is not scaled to operate in the 88 to 108 MHz FM band.

MisterMe 18-Jan-2012 4:00 AM

Quote:

Originally Posted by sc2dave (Post 19445)
Here it is.

Your template is really semi-fractal more than fractal. GroundUrMast's earlier comment was accurate. A fractal antenna is directional with the strongest reception perpendicular to the plane of the antenna. If you mount the antenna in the horizontal plane, then it will be in its least sensitive orientation. I would also expect its reception pattern to be high anisotropic, making it a poor choice for a moving vehicle. This does not even get into the fact that your truck's roof is a ground plane.

What this all gets down to is that you have a bad choice of antenna for a moving vehicle and that you want to mount it in the worst possible orientation. There is a reason why most automotive antennas are monopoles, that most automotive dipole antennas don't work well, and that nobody but nobody mounts automotive antennas in the horizontal plane.

sc2dave 18-Jan-2012 4:17 AM

Quote:

Originally Posted by MisterMe (Post 19449)
What this all gets down to is that you have a bad choice of antenna for a moving vehicle and that you want to mount it in the worst possible orientation. There is a reason why most automotive antennas are monopoles, that most automotive dipole antennas don't work well, and that nobody but nobody mounts automotive antennas in the horizontal plane.

What if i were to mount this on the back window of my truck,like this ;http://www.youtube.com/watch?v=Y-4a8pVdSOE go to 1:08

GroundUrMast 18-Jan-2012 6:41 AM

4NEC2 model of YouTube semi-fractal
 
1 Attachment(s)
CM
CE
SY WR=#16
SY DEX=0
SY PLS=0.0254
SY DESEGL=0.0254
SY DEPCGZ=0.1016
GW 1 5 DEX PLS/2 DEPCGZ/2 DEX (PLS/2)+cos(30)*DESEGL (DEPCGZ/2)+sin(30)*DESEGL WR
GW 2 5 DEX (PLS/2)+cos(30)*DESEGL (DEPCGZ/2)+sin(30)*DESEGL DEX (PLS/2)+cos(30)*DESEGL ((DEPCGZ/2)+sin(30)*DESEGL)+DESEGL WR
GW 3 5 DEX (PLS/2)+cos(30)*DESEGL ((DEPCGZ/2)+sin(30)*DESEGL)+DESEGL DEX ((PLS/2)+cos(30)*DESEGL)+(cos(-30)*DESEGL) (((DEPCGZ/2)+sin(30)*DESEGL)+DESEGL)+sin(-30)*DESEGL WR
GW 4 5 DEX ((PLS/2)+cos(30)*DESEGL)+(cos(-30)*DESEGL) (((DEPCGZ/2)+sin(30)*DESEGL)+DESEGL)+(sin(-30)*DESEGL) DEX (((PLS/2)+cos(30)*DESEGL)+cos(-30)*DESEGL)+cos(30)*DESEGL ((((DEPCGZ/2)+sin(30)*DESEGL)+DESEGL)+sin(-30)*DESEGL)+sin(30)*DESEGL WR
GW 5 5 DEX PLS/2 DEPCGZ/2 DEX (PLS/2)+cos(30)*DESEGL (DEPCGZ/2)-sin(30)*DESEGL WR
GW 6 5 DEX (PLS/2)+cos(30)*DESEGL (DEPCGZ/2)-sin(30)*DESEGL DEX (PLS/2)+cos(30)*DESEGL ((DEPCGZ/2)-sin(30)*DESEGL)-DESEGL WR
GW 7 5 DEX (PLS/2)+cos(30)*DESEGL ((DEPCGZ/2)-sin(30)*DESEGL)-DESEGL DEX ((PLS/2)+cos(30)*DESEGL)+(cos(-30)*DESEGL) (((DEPCGZ/2)-sin(30)*DESEGL)-DESEGL)-sin(-30)*DESEGL WR
GW 8 5 DEX ((PLS/2)+cos(30)*DESEGL)+(cos(-30)*DESEGL) (((DEPCGZ/2)-sin(30)*DESEGL)-DESEGL)-sin(-30)*DESEGL DEX (((PLS/2)+cos(30)*DESEGL)+cos(-30)*DESEGL)+cos(30)*DESEGL ((((DEPCGZ/2)-sin(30)*DESEGL)-DESEGL)-sin(-30)*DESEGL)-sin(30)*DESEGL WR
GW 9 10 DEX PLS/2 0 DEX PLS/2 DEPCGZ/2 WR
GX 9 001
GX 18 010
' GW 100 5 DEX PLS/-2 DEPCGZ/-2 DEX PLS/2 DEPCGZ/-2 WR
GW 100 5 DEX PLS/-2 0 DEX PLS/2 0 WR
GE 0
EK
EX 0 100 3 0 1 0 0 0
LD 5 0 0 0 58000000
GN -1
' FR 0 0 0 0 99.9 0
FR 0 111 0 0 85 10
' FR Freq Sweep choices in order of increasing calculation time (fm holl_ands):
' FR 0 0 0 0 470 0 ' Fixed Freq
' FR 0 29 0 0 470 12 ' Freq Sweep 470-806 every 12 MHz - OLD UHF BAND
' FR 0 34 0 0 410 12 ' Freq Sweep 410-806 every 12 MHz - Even Wider Sweep
' FR 0 39 0 0 470 6 ' Freq Sweep 470-698 every 6 MHz - PREFERRED FOR UHF
' FR 0 77 0 0 470 3 ' Freq Sweep 470-698 every 3 MHz
' FR 0 153 0 0 470 1.5 ' Freq Sweep 470-698 every 1.5 MHz
' FR 0 71 0 0 300 10 ' Freq Sweep 300-1000 every 10 MHz - WIDEBAND SWEEP
' FR Hi-VHF choices:
' FR 0 15 0 0 174 3 ' Freq Sweep 174-216 every 3 MHz
' FR 0 29 0 0 174 1.5 ' Freq Sweep 174-216 every 1.5 MHz - PREFERRED
' FR 0 45 0 0 162 1.5 ' Freq Sweep 162-228 every 1.5 MHz - Add +/- 12 MHz BW
' FR 0 27 0 0 189 1.5 ' Freq Sweep 186-228 every 1.5 MHz - SPECIAL
' FR 0 43 0 0 174 1 ' Freq Sweep 174-216 every 1 MHz - Hi-Rez
' FR 0 23 0 0 198 1 ' Freq Sweep 198-220 every 1 MHz - Hi-Rez - Ch13 SPECIAL
' FR 0 26 0 0 150 6 ' Freq Sweep 150-300 every 6 MHz - WIDEBAND SWEEP
' FR Lo-VHF choices:
' FR 0 19 0 0 54 3 ' Frequency Sweep every 3 MHz for Ch2-6 + FM
' FR 0 35 0 0 54 1 ' Frequency Sweep every 1 MHz for Ch2-6
' FR 0 36 0 0 75 1 ' Frequency Sweep every 1 MHz for Ch5 + Ch6 + FM
' FR 0 28 0 0 54 6 ' Wide Freq Sweep every 6 MHz for Ch2-13
' FR 0 64 0 0 54 12 ' Super Wide Freq Sweep 54-810 every 12 MHz
' RP choices in order of increasing calculation time:
' RP 0 1 1 1510 90 90 1 1 0 0 ' 1D Gain toward 0-deg Azimuth - SIDE GAIN
' RP 0 1 1 1510 90 0 1 1 0 0 ' 1D Gain toward 90-deg Azimuth - FORWARD GAIN
' RP 0 1 1 1510 90 180 1 1 0 0 ' 1D Gain toward 270-deg Azimuth - REVERSE GAIN
' RP 0 1 37 1510 90 0 1 5 0 0 ' 2D (Left only) Azimuthal Gain Slice
RP 0 1 73 1510 90 0 1 5 0 0 ' 2D Azimuthal Gain Slice - PREFERRED
' RP 0 73 1 1510 90 0 5 1 0 0 ' 2D Elevation Gain Slice
' RP 0 73 73 1510 90 0 5 5 0 0 ' 3D Lower Hemisphere reveals antenna (Fixed Freq)
' RP 0 285 73 1510 90 0 5 5 0 0 ' 3D Full Coverage obscures antenna (Fixed Freq)
EN

GroundUrMast 18-Jan-2012 6:45 AM

1 Attachment(s)
As I suspected, the "antenna" shown on Youtube and in the drawing has extremely high SWR (in excess of 2000:1) in the FM broadcast band which means it will simply reflect almost all of the signal back into the air, delivering far less signal to the radio than a stock antenna. The antenna does not come close to usable until you look at frequencies in the upper cell-phone bands and terrestrial microwave region... but is still far from optimum in design.

At frequencies above 1000 MHz, the antenna shows modest gain to the front and rear.

MisterMe 18-Jan-2012 6:13 PM

Quote:

Originally Posted by sc2dave (Post 19450)
What if i were to mount this on the back window of my truck,like this ;http://www.youtube.com/watch?v=Y-4a8pVdSOE go to 1:08

I note that the semi-fractal antenna in the YouTube video is a Mobile DTV antenna, not an FM antenna. The FM radio band is adjacent to NTSC/ATSC Channel 6. GroundUrMast has done an excellent job simulating this antenna. I do not see it as the optimum choice for MDTV. Because it is intended to receive digital transmissions, it should be more effective in this role than it would be if used to receive analog signals such and analog FM.

GroundUrMast 19-Jan-2012 6:16 AM

Comparison of 1/4 vs 5/8 radiation patterns
 
1 Attachment(s)
Here is a 3D model view of the radiation patterns of the common 1/4 wave whip compared to the theoretical radiation pattern of a 5/8 wavelength base loaded whip.

Both are simulated over perfect ground... which your fender will is not.

Here is the 4NEC2 code for the 5/8 WL model:

CM Base loaded 5/8 wave vertical whip antenna over a perfect ground plane
CE
SY WR=#14
SY DEL=1.868521 'Element Section Length in Meters
SY COIL=6.125e-7 'Load Inductance, Henrys
GW 1 30 0 0 0 0 0 DEL WR
GE 1
LD 5 0 0 0 1390000 'Stainless Steel
LD 0 1 2 2 0 COIL
GN 1
EK
EX 0 1 1 0 1 0 0
FR 0 0 0 0 100 0
RP 0 285 73 1510 90 0 5 5 0 0
EN


Here is the 4NEC2 code for the 1/4 WL model:

CM Simple quarter wave vertical whip antenna over a perfect ground plane
CE
SY WR=#14
SY DEL=0.755 'Element length in Meters
GW 1 17 0 0 0 0 0 DEL WR
GE 1
LD 5 0 0 0 1390000 'Stainless Steel
GN 1
EK
EX 0 1 1 0 1 0 0
FR 0 0 0 0 100 0
RP 0 285 73 1510 90 0 5 5 0 0
EN

sc2dave 20-Jan-2012 7:06 PM

Sorry,all of these letters and numbers are 100% Greek to me.


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