Forum Jump: 
 1Likes
  • 1 Post By ProjectSHO89
 
Thread Tools
post #1 of 25 Old 04-08-2015, 03:06 PM - Thread Starter
Member
 
JoeWS's Avatar
 
Join Date: Jun 2008
Location: Haddonfield, NJ
Posts: 68
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 8 Post(s)
Liked: 12
Low VHF Dipole - For PHILLY Channels 2 and 6

I want to add a lo-VHF dipole for channels 2 and 6 in Philly. Both signals are very strong but not very reliable with my RCA ANT751. Can anyone give me some tips on size, construction ideas, etc. My ANT751 is roof mounted but I was thinking of putting the dipole in the attic since both channels are over 60 NM(db) on my TvFool Report (and only 15 miles away). I use a CM 7777 preamp so I was thinking of joining the signals after the power supply to minimize signal loss from the rooftop. Comments?
JoeWS is offline  
Sponsored Links
Advertisement
 
post #2 of 25 Old 04-08-2015, 03:18 PM
AVS Forum Special Member
 
Calaveras's Avatar
 
Join Date: Jan 2007
Location: Mountain Ranch
Posts: 5,793
Mentioned: 20 Post(s)
Tagged: 0 Thread(s)
Quoted: 1636 Post(s)
Liked: 348
With those high noise margins even a simple dipole cut for the middle of the band should receive them as long as multipath isn't an issue. Don't bother with a preamp. The environmental noise is so high on low VHF and the coax loss is so low that a preamp will do nothing. I removed my preamp on low VHF even though I have 575' of coax (not RG-6) and there was no difference even on a station 119 miles away with a negative noise margin shown by TV Fool.

A dipole cut for the middle of low VHF is 78" long.
Calaveras is offline  
post #3 of 25 Old 04-08-2015, 07:59 PM
AVS Forum Special Member
 
rabbit73's Avatar
 
Join Date: Apr 2008
Location: S.E. VA
Posts: 3,050
Mentioned: 19 Post(s)
Tagged: 0 Thread(s)
Quoted: 775 Post(s)
Liked: 318
Try a folded dipole antenna made from 300 ohm twin-lead in the attic. As Calaveras said, 2 and 6 are very strong, but so is the noise level from electrical interference on VHF-low (2-6).

Sometimes the noise level is so high that it overpowers the TV signals and reduces the SNR (Signal to Noise Ratio) to the point that reception is impossible.

So the idea is to improve that ratio so that the signal is stronger than the noise, by constructing an antenna for the specific channel desired.

Make a folded dipole for channel 6 and test it in the attic.



CH 6, 82 - 88 MHz Center frequency = 85 MHz, 5540/85 = 65.2 inches for folded dipole

65.2 x 105% = 68.5 inches for reflector, if used to make 2 element beam to increase gain and reject signals from the rear to improve SNR

1/4 wave = 32.6 inches for driven element to reflector spacing
.................................................. ................................................
CH 4, 66 -72 MHz Center frequency = 69 MHz, 5540/69 = 80.3 inches for folded dipole
.................................................. .............................................

CH 2, 54 - 60 MHz Center frequency = 57 MHz, 5540/57 = 97.2 inches for folded dipole

97.2 x 105 % = 102.1 inches for reflector, if used to make a 2 element beam to increase gain and reject signals from the rear to improve SNR

1/4 wave = 49 inches, for driven element to reflector spacing
.................................................. ................................................
.
If it works well, try one for channel 2. It might be possible that a folded dipole cut for channel 4 will get 2 and 6 OK.

If you are still having problems with one channel, add a reflector wire that is 5% longer, and spaced 0.2 to 0.25 wave behind it. This will add a little gain and improve the SNR, because it will reject some of the noise from the rear
.
http://users.wfu.edu/matthews/misc/dipole.html

Your DIY antenna can be combined using a HLSJ.

If you are still having problems with 2 and 6, you would need a larger antenna in the attic or a very large antenna outside.
Attached Thumbnails
Click image for larger version

Name:	Folded Dipole.jpg
Views:	4056
Size:	59.4 KB
ID:	656713  

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
www.megalithia.com/elect/aerialsite/dttpoorman.html

Last edited by rabbit73; 04-23-2015 at 05:03 PM.
rabbit73 is offline  
Sponsored Links
Advertisement
 
post #4 of 25 Old 04-08-2015, 08:14 PM
AVS Forum Special Member
 
holl_ands's Avatar
 
Join Date: Jun 2004
Location: San Diego, CA
Posts: 5,874
Mentioned: 28 Post(s)
Tagged: 1 Thread(s)
Quoted: 843 Post(s)
Liked: 412
Folded Dipole using either Twin-Lead (typ. AWG24 wire gauge)...or even 1/2-in O.D. Copper Tubing is TOO SKINNY to provide good SWR performance on BOTH Ch2 and Ch6.....and Stick Dipole (not Folded) is even worse:
http://imageevent.com/holl_ands/loops/vhffoldeddipole
High SWR can not only result in an SWR NULL being located on the END of a Coax line, but can ALSO degrade the digital signal as reflected signals bounce up and down the Coax, resulting in Intersymbol Interference.

Fol. Ch2-6 Hourglass-Loop Antenna provides about 3-4 dBi Raw Gain (2-3 dB more than Dipole) with ACCEPTABLE SWR from Ch2-6 (and even thru FM Band):
http://imageevent.com/holl_ands/loop...mhourglassloop
Wire Size is NOT all that important, but I would recommend using AT LEAST AWG10 (house electrical wire) and preferably 1/4 to 1/2-in Copper Tubing bent to shape (or Pipe with fittings)...simply hang it from the rafters in the Attic.

Lo-VHF/Hi-VHF+UHF Combiner (aka HLSJ or ZHLSJ) is used on the OUTPUT of Preamp (if used) so that Ch2-6 signals are NOT amplified. Mfr description say that DC PASS is ONLY on the Lo-VHF Port....so Power insertion to Preamp needs to be between Preamp and HLSJ. [I don't recall whether this is true or not....]

Last edited by holl_ands; 04-10-2015 at 11:40 AM.
holl_ands is online now  
post #5 of 25 Old 04-09-2015, 06:50 AM
AVS Forum Addicted Member
 
Ratman's Avatar
 
Join Date: May 2002
Location: Collingswood, N.J.
Posts: 18,369
Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1870 Post(s)
Liked: 1650
JoeWS:

I'm a couple miles away from you. I just added a simple dipole (fully extended) I had laying around to the CM4228 in my attic. It works fine for all the Philly VHF channels 2 through 12.

http://www.amazon.com/Magnavox-MANT-.../dp/B00006JPR2
Ratman is offline  
post #6 of 25 Old 04-09-2015, 11:49 AM
AVS Forum Special Member
 
holl_ands's Avatar
 
Join Date: Jun 2004
Location: San Diego, CA
Posts: 5,874
Mentioned: 28 Post(s)
Tagged: 1 Thread(s)
Quoted: 843 Post(s)
Liked: 412
Since it's not possible for us to measure how strong your Ch2/6 signals are in YOUR Attic (some are OK and some are pretty bad, esp in VHF Band), you'll just have to TRY some of these ideas and see if you need something "better". Avoid laying the Antenna directly on the Floor of the Attic....SUSPEND it from the Rafters....and LOCATION, LOCATION, LOCATION....
holl_ands is online now  
post #7 of 25 Old 04-09-2015, 12:09 PM
AVS Forum Addicted Member
 
Ratman's Avatar
 
Join Date: May 2002
Location: Collingswood, N.J.
Posts: 18,369
Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1870 Post(s)
Liked: 1650
Just trying to provide the OP with a POSSIBLE simple solution since he's only a FEW miles away from MY location.
YOU never know... his attic MAY be similar to mine. Both OLD towns and possibly similar construction.

And yes... both my 4228 and dipole are SUSPENDED from the rafters.

Last edited by Ratman; 04-09-2015 at 12:14 PM.
Ratman is offline  
post #8 of 25 Old 04-09-2015, 12:47 PM
AVS Forum Special Member
 
rabbit73's Avatar
 
Join Date: Apr 2008
Location: S.E. VA
Posts: 3,050
Mentioned: 19 Post(s)
Tagged: 0 Thread(s)
Quoted: 775 Post(s)
Liked: 318
Antennacraft used to make a channel 2-6 antenna the Y5-2-6, but it is hard to find:

http://www.google.com/?gws_rd=ssl#q=y5-2-6+antennacraft

http://www.radioshack.com/antennacraft-y526-80-cut-to-band-hdtv-antenna/1500047.html#.VSbGZTh0xLM

http://www.antennacraft.net/pdfs/Y5-2-6.pdf

The only other alternatives are a Winegard HD8200U or a DIY LPDA for 2-6:
http://www.solidsignal.com/pview.asp?p=hd8200u

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
www.megalithia.com/elect/aerialsite/dttpoorman.html

Last edited by rabbit73; 04-09-2015 at 01:18 PM.
rabbit73 is offline  
post #9 of 25 Old 04-10-2015, 11:46 AM
AVS Forum Special Member
 
holl_ands's Avatar
 
Join Date: Jun 2004
Location: San Diego, CA
Posts: 5,874
Mentioned: 28 Post(s)
Tagged: 1 Thread(s)
Quoted: 843 Post(s)
Liked: 412
FYI: A-C Y5-2-6 has High SWR on Ch2, which may or may not cause problems.....it's more of a Ch3-6 Antenna...with Raw Gain = 6-8 dBi...and it's a BIG 80"L x 112"W:
http://imageevent.com/holl_ands/yagis/ch26acy5265elyagi
holl_ands is online now  
post #10 of 25 Old 04-21-2015, 11:10 AM
AVS Forum Special Member
 
Andrew K's Avatar
 
Join Date: Jul 2006
Location: Akron, Ohio
Posts: 1,744
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 1024 Post(s)
Liked: 219
Does anyone know what the gain would be if I used the AntennaCraft FM-6 for channel 2? Is it possible to catch e-skip using an FM-6 on channel 2??

Last edited by Andrew K; 04-21-2015 at 11:13 AM.
Andrew K is offline  
post #11 of 25 Old 04-21-2015, 01:12 PM
AVS Forum Special Member
 
holl_ands's Avatar
 
Join Date: Jun 2004
Location: San Diego, CA
Posts: 5,874
Mentioned: 28 Post(s)
Tagged: 1 Thread(s)
Quoted: 843 Post(s)
Liked: 412
I converted K6STI's model for Antennacraft FM-6 into a 4nec2 File and ran nikiml's EVAL function on it with fol. Results. Not much Raw Gain on Ch2 and SWR is astronomical, which may or may NOT cause problems...esp. if NULL on the Coax coincides with the END. Note that I had to do a minor revision for the Simulated SOURCE wire and do AUTOSEG(15) to work in 4nec2:
http://ham-radio.com/k6sti/fm6.htm
Results and a copy of the 4nec2 file are found at bottom of this post.
If you are wondering whether it was POSSIBLE to have received Ch2 via your FM-6....YES, it's possible since some E-Skip signals are quite strong....but it isn't the Tool-of-Choice for a serious TV DX'er.

For comparison, A-C Y5-2-6 only has 6 dBi Raw Gain on Ch2 with Excessive SWR = 8. This should be adequate for simple, single-reflection E-Skip scenarios with strong signals. A lot more re E-Skip TV and FM DX can be found at Todd Emslie's Website (he's in Australia, where Trans-Equatorial and Tropo Duct propagation modes are also frequently available):
http://home.iprimus.com.au/toddemsli...radic%20E.html

IF you are looking for weaker MULTI-HOP E-Skip, Todd Emslie recommends a HIGH-GAIN Antenna, such as in your case, a SINGLE CHANNEL (Ch2) YAGI:
http://www.wadeantenna.com/ius/produ...0LOG_N0515.pdf
http://home.iprimus.com.au/toddemslie/dx.html
More info, esp. re. Multi-Hop E-Skip:
http://www.dxfm.com/content/propagation.htm
http://en.wikipedia.org/wiki/TV_and_FM_DX

The fol. 5 and 6-Element Yagi DIY Projects have been Optimized for Ch3. For Ch2, increase ALL Dimensions by the Rescale Factor = 66/60 = 1.1. Which means make ALL Dimensions 10% BIGGER.
http://imageevent.com/holl_ands/yagis/ch35elfdyagiopt
http://imageevent.com/holl_ands/yagis/ch36elfdyagiopt


FM-6 EVAL RESULTS:
Code:
Input file : C:\Python33\FM6_K6STI-holl_ands.nec
Freq sweeps: [(54, 3, 19)]
Autosegmentation: NO

         --- Gain ---              -- Ratios -- -- Impedance --
   Freq    Raw    Net   SWR  BeamW    F/R    F/B    Real    Imag  AGT  corr
==========================================================================
   54.0   1.01 -21.00 633.60  75.5   1.08   1.08    0.48  -17.17 0.92 -0.37  ' Ch2
   57.0   1.80 -19.02 481.38  74.4   1.25   1.25    0.62   -0.79 0.92 -0.37  ' Ch2
   60.0   2.52 -16.94 350.94  73.6   1.42   1.42    0.86   16.46 0.92 -0.37  ' Ch2
   63.0   2.90 -14.98 243.83  73.1   1.58   1.58    1.25   35.22 0.97 -0.12
   66.0   3.41 -12.68 160.66  72.8   1.70   1.70    1.93   56.42 0.97 -0.12
   69.0   3.78 -10.30 100.31  72.7   1.75   1.75    3.21   81.53 0.97 -0.12
   72.0   3.80  -8.09  59.80  73.2   1.58   1.58    5.73  113.05 1.00  0.02
   75.0   3.62  -6.04  34.98  74.2   0.92   0.92   10.89  155.77 1.00  0.02
   78.0   2.56  -5.14  21.54  77.6  -1.09  -1.09   21.54  221.44 1.00  0.02
   81.0  -1.80  -8.27  15.68  80.7  -6.98  -6.98   47.53  363.72 1.12  0.50

   84.0   7.26   4.59   5.19  59.4   4.16   4.16 1134.46  675.50 1.12  0.50 ' Ch6

   87.0   8.15   8.07   1.31  63.2  15.52  15.52  318.78  -81.99 1.12  0.50
   90.0   8.24   8.24   1.03  64.2  20.26  20.26  291.85    0.60 1.04  0.16
   93.0   8.17   8.15   1.14  64.0  19.11  19.11  338.32  -17.67 1.04  0.16
   96.0   8.24   8.05   1.52  63.2  17.26  17.26  349.56 -127.10 1.04  0.16
   99.0   8.91   8.24   2.21  61.9  15.30  15.30  241.24 -210.98 0.92 -0.36
  102.0   9.08   7.75   3.12  60.0  13.28  13.28  137.37 -180.97 0.92 -0.36
  105.0   9.14   7.87   3.02  57.1  11.49  11.49  113.85 -107.41 0.92 -0.36
  108.0   8.92   7.64   3.05  53.1  10.67  10.67  234.86 -304.42 0.92 -0.36
Press any key to continue . . .


FM-6
4nec2 FILE:
Code:
CM Antennacraft FM6, original model by K6STI, converted to 4nec2 by holl_ands, 21Apr2015
CM Free Space, 98 MHz, 39 6063-T832 wires, inches
CM holl_ands mods: Add FR/RP Cards & Rsrc.  AUTOSEG(31), NO Errors or Warnings, AGT=1.0.
CM Note that K6STI's model used Element Diameter, which needed to be changed to Radius for 4nec2.
CM
CMD--EVAL --auto-segmentation=0 --char-impedance=300 --num-cores=6
CMD--EVAL -s(54,3,19)
CE
' New SYmbol Statement: Adjust Rsrc for AGT=1.0
SY Rsrc=0.2654
'
' Fol. are as provided in K6STI's original file:
SY r = 1.875 / 2            ' rivet half-spacing
SY yo = r * (1 - 3.375 / 6.1875)    ' y at outer phasing-line bends
SY yi = r * (1 - 3.875 / 6.1875)    ' y at inner bends
SY x1 = -17.75 + 3.375              ' x at first phasing-line bend, rear line
SY x2 = -17.75 + 3.875              ' x at second bend
SY x3 = -5.375 - 3.875              ' x at third bend
SY x4 = -5.375 - 3.375              ' x at fourth bend
SY x5 = -5.375 + 3.375              ' x at first phasing-line bend, forward line
SY x6 = -5.375 + 3.875              ' x at second bend
SY x7 = 7 - 3.875                   ' x at third bend
SY x8 = 7 - 3.375                   ' x at fourth bend
SY s1 = .375 / 2                    ' rear phasing-line crossover half-spacing
SY s2 = .5625 / 2                   ' front phasing-line crossover half-spacing
SY t = -.75                ' terminals
SY f = -1.75                ' feedpoint
'
GW 1    17    -33.75    -33         0       -33.75     -2         0        0.375/2
GW 2    3     -33.75     -2         0       -33.75      2         0        1.1/2
GW 3    17    -33.75      2         0       -33.75     33         0        0.375/2
GW 4    15    -17.75    -29.25      0       -17.75     -r         0        0.375/2
GW 5    15    -17.75      r         0       -17.75     29.25      0        0.375/2
GW 6    15    -5.375   -28.125     0        -5.375    -r         0        0.375/2
GW 7    15    -5.375     r         0        -5.375    28.125     0        0.375/2
GW 8    11     7       -21.6875    0         7        -r         0        0.375/2
GW 9    1      7        -r         0         7        -r         t        0.125/2  ' zinc
'
' Had to revise Simulated SOURCE:
GW 10    1      7        -r         t         7         r         t         Rsrc
' GW 10    2      7        -r         t         7         0         f         #18   ' copper
' GW 11    2      7         r         t         7         0         f         #18   ' copper
'
GW 12    1      7         r         0         7         r         t        0.125/2  ' zinc
GW 13    11     7         r         0         7        21.6875    0        0.375/2
GW 14    13    10.25    -25         0        10.25     -2         0        0.375/2
GW 15    3     10.25     -2         0        10.25      2         0        1.1/2
GW 16    13    10.25      2         0        10.25     25         0        0.375/2
GW 17    13    33.75    -25         0        33.75     -2         0        0.375/2
GW 18    3     33.75     -2         0        33.75      2         0        1.1/2
GW 19    13    33.75      2         0        33.75     25         0        0.375/2
GW 20    3    -17.75      r         0         x1       yo         0         .11/2
GW 21    1      x1       yo         0         x2       yi       -s1         .11/2
GW 22    3      x2       yi       -s1         x3      -yi       -s1         .11/2
GW 23    1      x3      -yi       -s1         x4      -yo         0         .11/2
GW 24    3      x4      -yo         0        -5.375    -r         0         .11/2
GW 25    3    -17.75     -r         0         x1      -yo         0         .11/2
GW 26    1      x1      -yo         0         x2      -yi        s1         .11/2
GW 27    3      x2      -yi        s1         x3       yi        s1         .11/2
GW 28    1      x3       yi        s1         x4       yo         0         .11/2
GW 29    3      x4       yo         0        -5.375     r         0         .11/2
GW 30    3     -5.375     r         0         x5       yo         0         .11/2
GW 31    1      x5       yo         0         x6       yi        s2         .11/2
GW 32    3      x6       yi        s2         x7      -yi        s2         .11/2
GW 33    1      x7      -yi        s2         x8      -yo         0         .11/2
GW 34    3      x8      -yo         0         7        -r         0         .11/2
GW 35    3     -5.375    -r         0         x5      -yo         0         .11/2
GW 36    1      x5      -yo         0         x6      -yi       -s2         .11/2
GW 37    3      x6      -yi       -s2         x7       yi       -s2         .11/2
GW 38    1      x7       yi       -s2         x8       yo         0         .11/2
GW 39    3      x8       yo         0         7         r         0         .11/2
'
GS 0 0 0.0254            ' All in inches.
GE 0
GN -1
EK 0
LD 5 0 0 0 2.00e7        ' Aluminum Elements
'
EX 0 10 1 0 1 0
'
' 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 25 0 0 192 1        ' Freq Sweep 192-216 every 1 MHz (Special for F/B)
' 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 43 0 0 174 1        ' Freq Sweep 174-216 every 1 MHz - Hi-Rez
' 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

Last edited by holl_ands; 04-21-2015 at 02:18 PM.
holl_ands is online now  
post #12 of 25 Old 04-21-2015, 01:14 PM
Member
 
Join Date: Feb 2015
Posts: 69
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 23 Post(s)
Liked: 14
try this!

http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

All animals are equal, but some are more equal than others
nextgenfmradio is offline  
post #13 of 25 Old 04-21-2015, 02:31 PM
AVS Forum Special Member
 
Andrew K's Avatar
 
Join Date: Jul 2006
Location: Akron, Ohio
Posts: 1,744
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 1024 Post(s)
Liked: 219
Quote:
Originally Posted by holl_ands View Post
I converted K6STI's model for Antennacraft FM-6 into a 4nec2 File and ran nikiml's EVAL function on it with fol. Results. Not much Raw Gain on Ch2 and SWR is astronomical, which may or may NOT cause problems...esp. if NULL on the Coax coincides with the END.
Thanks, that confirms that putting that bulky antenna outside is not worth the effort to pull in those VHF-low stations. Perhaps just a simple dipole is better. Is a dipole omnidirectional? I just want to set the antenna outside and put my tuner on auto-pilot while I'm away from home or asleep to see if it decodes any e-skip.
Andrew K is offline  
post #14 of 25 Old 04-21-2015, 04:21 PM
AVS Forum Special Member
 
rabbit73's Avatar
 
Join Date: Apr 2008
Location: S.E. VA
Posts: 3,050
Mentioned: 19 Post(s)
Tagged: 0 Thread(s)
Quoted: 775 Post(s)
Liked: 318
A dipole antenna is bi-directional.


Attached Thumbnails
Click image for larger version

Name:	Dipole Pattern2a_1.jpg
Views:	3667
Size:	97.4 KB
ID:	682553  

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
www.megalithia.com/elect/aerialsite/dttpoorman.html

Last edited by rabbit73; 04-23-2015 at 04:55 PM.
rabbit73 is offline  
post #15 of 25 Old 04-21-2015, 04:36 PM
Member
 
Join Date: Feb 2015
Posts: 69
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 23 Post(s)
Liked: 14
Red face

Quote:
Originally Posted by rabbit73 View Post
A dipole antenna is bi-directional.

that's true, i'll pick up the stations 180 degrees the other direction

All animals are equal, but some are more equal than others
nextgenfmradio is offline  
post #16 of 25 Old 04-22-2015, 03:06 PM
AVS Forum Special Member
 
holl_ands's Avatar
 
Join Date: Jun 2004
Location: San Diego, CA
Posts: 5,874
Mentioned: 28 Post(s)
Tagged: 1 Thread(s)
Quoted: 843 Post(s)
Liked: 412
In order to provide low SWR on Ch2, a simple Stick Dipole formed from 3/8-in Elements needs to be 95-in Total Length (well beyond the limit of Rabbit-Ears), using a 75-ohm DIRECT Coax connection...if using 300:75-ohm Transformer Balun, the SWR gets worse and there will be some Loss in the Balun:
http://imageevent.com/holl_ands/dipoles/vhfstickdipoles [Images 10-12]
FAT Elements are essential to an acceptable SWR, such as Refrigerator Tubing or Straight Copper Tubing [Careful, Tube sizes are for the INSIDE Diameter].

Twin-lead (AWG24 wire size with 0.5-in Separation) [Or two parallel wires, shorten Length by amount of additional Separtion] can be formed into a Folded Dipole, providing acceptable SWR across Ch2 if the Total ELECTRICAL Length (in free space) is 99.25-in [For Physical Length...shorten by Velocity Factor (typ. 0.8 to 0.9) if using Plastic Twin-Lead]:
http://imageevent.com/holl_ands/loops/vhffoldeddipole

For more Raw Gain than a 2 dBi Dipole, the fol. Ch2 Hourglass-Loop + 3RR (simple DIY Project) provides Raw Gain = 9.7 to 9.0 dB....and can be constructed using AWG10 or AWG12 "House" Electrical Wire (there are THREE wires in NM-B 10-2 or 12-2) over a PVC or Wood (Dry Attic Only) Frame:
http://imageevent.com/holl_ands/loop...ssloopreflrods

Last edited by holl_ands; 04-22-2015 at 03:17 PM.
holl_ands is online now  
post #17 of 25 Old 04-23-2015, 01:38 PM
AVS Forum Special Member
 
holl_ands's Avatar
 
Join Date: Jun 2004
Location: San Diego, CA
Posts: 5,874
Mentioned: 28 Post(s)
Tagged: 1 Thread(s)
Quoted: 843 Post(s)
Liked: 412
rabbit73:
In your above post #3 you incorrectly DIVIDED by the Velocity Factor (hence Multiply by 1.05) rather than Multiply by the Velocity Factor itself, as correctly shown in the DIY Folded Dipole link.

Also, a Velocity Factor of 0.95 is no where NEAR what I see in MOST Twinlead cables....fol. from a post citing (1994) Belden Catalog:
9085 300 4.5 80% 1.4 brown cellular poly (foam?)
8285 300 4.6 73.3% 1.4 regular twinlead w cell poly over it
8230 300 3.6 80% 1.1 regular twinlead, cu coated steel
8225 300 3.6 80% 1.1 classic regular twinlead (web)
8275 300 4.6 80% 1.0 round poly with wires, foam core

Some additional data points for available 300-ohm Twinlead:
http://support.radioshack.com/suppor...oc37/37499.htm [VF = 0.733 for RS 150-1174, hopefully obsolete]
http://support.radioshack.com/suppor...oc17/17128.htm [VF = 0.82 for RS 150-1153]
http://www.vk2zay.net/article/121 [VF = 0.825, 0.833]
http://www.packetradio.com/catalog/i...oducts_id=1768 [VF = 0.85]
http://www.packetradio.com/catalog/i...oducts_id=1776 [VF = 0.88]

And I've seen antique Radio-Shack Twinlead with reported VF - 0.77. (Thankfully obsolete many decades ago.]
And HDTVPrimer Glossary indicates that some Twinlead had as low as VF=0.66. [Ditto....hopefully....]

Last edited by holl_ands; 04-23-2015 at 01:54 PM.
holl_ands is online now  
post #18 of 25 Old 04-23-2015, 01:48 PM
AVS Forum Addicted Member
 
Ratman's Avatar
 
Join Date: May 2002
Location: Collingswood, N.J.
Posts: 18,369
Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1870 Post(s)
Liked: 1650
I wonder how (or if) JoeWS worked out his issues?
Ratman is offline  
post #19 of 25 Old 04-23-2015, 04:19 PM
AVS Forum Special Member
 
rabbit73's Avatar
 
Join Date: Apr 2008
Location: S.E. VA
Posts: 3,050
Mentioned: 19 Post(s)
Tagged: 0 Thread(s)
Quoted: 775 Post(s)
Liked: 318
Quote:
Originally Posted by holl_ands View Post
rabbit73:
In your above post #3 you incorrectly DIVIDED by the Velocity Factor (hence Multiply by 1.05) rather than Multiply by the Velocity Factor itself, as correctly shown in the DIY Folded Dipole link.
That's not for the VF, it's for the reflector length. I will edit that post for clarity; thanks for calling it to my attention.
5540/12 = 461.7
VF compensation already incorporated in constant 5540 for answer in inches.
I'm a ham, so I don't do it the way he does.


It works for me.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
www.megalithia.com/elect/aerialsite/dttpoorman.html

Last edited by rabbit73; 04-23-2015 at 05:07 PM.
rabbit73 is offline  
post #20 of 25 Old 05-01-2015, 05:35 AM - Thread Starter
Member
 
JoeWS's Avatar
 
Join Date: Jun 2008
Location: Haddonfield, NJ
Posts: 68
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 8 Post(s)
Liked: 12
Ratman: Didn't find the time yet to tackle this issue. I plan to try a twinlead dipole first, cut to Channel 4 and see if it works. I'll report back.
JoeWS is offline  
post #21 of 25 Old 05-01-2015, 03:17 PM
AVS Forum Special Member
 
AntAltMike's Avatar
 
Join Date: Nov 2001
Location: College Park, MD
Posts: 3,979
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 169 Post(s)
Liked: 84
Quote:
Originally Posted by nextgenfmradio View Post
that's true, i'll pick up the stations 180 degrees the other direction
You'll also pick up reflections of your intended signals with equal efficiency...
AntAltMike is offline  
post #22 of 25 Old 05-05-2015, 08:56 AM
Advanced Member
 
ADTech's Avatar
 
Join Date: Jun 2010
Location: St Louis
Posts: 793
Mentioned: 4 Post(s)
Tagged: 0 Thread(s)
Quoted: 175 Post(s)
Liked: 89
Mike's point seems to have been missed. He was pointing out that, since the dipole is bidirectional, it has no ability to reject undesired signals from the rear which may otherwise cause destructive interference.

Tech support for Antennas Direct
ADTech is offline  
post #23 of 25 Old 12-22-2018, 08:53 PM
Newbie
 
ravi132's Avatar
 
Join Date: Jul 2002
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Liked: 11
Sorry to bump up an old thread, but does anyone have a suggestion for a low VHF antenna for channels 2 and 6 in Philly? Smaller/simpler the better if possible.
ravi132 is offline  
post #24 of 25 Old 12-23-2018, 06:30 AM
AVS Forum Special Member
 
ProjectSHO89's Avatar
 
Join Date: Feb 2009
Posts: 3,278
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 335 Post(s)
Liked: 155
"Smaller/Simpler" and low-VHF are generally mutually exclusive.
Calaveras likes this.
ProjectSHO89 is offline  
post #25 of 25 Old 12-23-2018, 06:35 AM
Super Moderator
 
DrDon's Avatar
 
Join Date: Mar 2002
Location: Metro Detroit, Tampa Bay
Posts: 16,196
Mentioned: 25 Post(s)
Tagged: 0 Thread(s)
Quoted: 1919 Post(s)
Liked: 3625
Lots of variables. See the Antenna Help sticky above and post all that information here.

Walking the fine line between jaw-dropping and a plain ol' yawn.
DrDon is offline  
Sponsored Links
Advertisement
 
Reply HDTV Technical



Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off