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Author Topic: Local 75M Dipole Actual Test: 30' vs: 70' High  (Read 68395 times)
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K1JJ
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« on: October 01, 2005, 12:36:57 PM »

For years, I've heard that for 75M, dipoles that are 25'-30' high are the best for close-in, local work.  So, I wanted to prove or disprove that idea here.

I put up a new dipole at about 70'. Reasonably flat and broadside NE/SW. Fed with hardline.  I wanted it away from the other antennas, so had to locate it within 100' of the street power lines. During the day, it has an S9 power line noise. At night the noise is masked by the strong signals. The other antennas farther than 300' from the street lines are pretty quiet.

So, I decided to set up another "quieter" dipole in the woods, about 500' from the lines. It is also fed with hardline, but is only 30' high - flat top. Same NE/SW directions. This antenna is absolutely noise quiet and a good receiving antenna during the day. Isolated by itself.

Now, for the actual comparisons over a week's time:

I did a lot of receive A/B tests. For both night and day, the LOCAL guys within 100 miles are usually about 2db! louder on the higher dipole at 70' !!   From 100-300 miles they are about 4-5db louder on the higher one and the W4's/W8's at 500+ miles are about 6-9 db louder on the high dipole.

Even guys that are 30 miles or LESS away are NOT louder on the lower 30' dipole. So, the 90 degree "zenith" beam idea seems to fall apart. At best, the real close in guys are within 1 db, but that's it.

Modeling shows the low dipole to have a vertical taleoff peak at about 88 degrees - high angle. While the 70' dipole peaks around 60 degrees - medium angle.

It might be ground losses of the lower dipole that are making this extra difference in addition to the takeoff angle.

So, my conclusion is that so called high angle "zenith" beams are not optimum, even for close in work. I was told by an antenna guru that the angles from 70-90 degrees didn't produce peak propagation and was wasted on 75M  - and that ~ 50-65 degrees is what counts for both local and trips out to 600 miles or so.

So, bottom line is what most of us have always suspected....  a dipole at 55'-80' is a great height for all around use, if you can pull it off.

Just wanted to shed some light on the old, "LOW dipole - LOCAL strapper" folklore. Your results may vary depending upon location, slope and ground conductivity, etc. 

I still plan to keep the 30' dipole in use due to its excellent quiet receiving ability. During the day it can hear an S1 signal, whereas, the high dipole is masked by the S9 line noise. They all have their purposes, just like a Gotham vertical.

73,
Tom, K1JJ
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« Reply #1 on: October 01, 2005, 04:59:09 PM »

In the real world, those low antennas just don't do the job. Modeling is nice but......
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« Reply #2 on: October 01, 2005, 07:45:07 PM »

In the real world, those low antennas just don't do the job. Modeling is nice but......

True, Dave. Antenna modeling works FB except for antennas below 1/4 wavelength high. Most software makes ground a perfect mirror reflection and the ground losses disappear. Bogus in the real whirl.

There was a time when I believed that a low dipole at 12' on 75M had huge high angle gain... according to the modeling anyway. I tried it and it was deaf.. :-)

NEC4 is a program that will give accurate results at low heights I understand, but costs $$.

But anyway, I'm finding one ADVANTAGE of the low 30' dipole-  the lower angle attenuates  the Wally and Richard QRM.  By selecting that antenna on receive the locals are still strong, but the 500+ miles stations drop 10db or so. So, besides killing the power line noise it is a decent local selector.

T
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« Reply #3 on: October 01, 2005, 07:51:01 PM »

I have to wonder what Wally and Richard are using for sky hooks. They aren't really too strong here in NJ. Funny how they seem to hop into area further Northeast. Maybe they have a few clipleads over to the old Chevy with no wheels in the front yard.
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« Reply #4 on: October 01, 2005, 08:08:13 PM »

I have to wonder what Wally and Richard are using for sky hooks. They aren't really too strong here in NJ. Funny how they seem to hop into area further Northeast. Maybe they have a few clipleads over to the old Chevy with no wheels in the front yard.

Well, if it's any clue, I remember one night hearing one of them discussing his HV supply being mounted remotely in the bathroom, on the other side of the wall... Grin  No kidding.

Optimum propagation angle selection has always been a debate. A lot depends upon where we are in the solar cycle and time of year. I've seen the 75M European angles vary from as low as 20 degrees to as high as 60 degrees on some nights. ie, Sometimes the low dipoles work as well as the high quads and then some nights the the low dipoles are deaf. Same for local signals during the day. Many times they are low angle due to the D attenuation.   The Tron [300 miles away]  is tremendously loud when he switches on his wire array in the daytime and I do the same to the NE.  With lower dipoles we drop off -15db between us. But at night, many times the low dipole is best between us cuz of the higher skip layer.

As you know, the best solution is to have a low and high antenna to select. Everything else is a compromise.  Vertical angle is most important for 20db differences sometimes, whereas forward gain is maybe 5-7db in the best cases.

T
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« Reply #5 on: October 03, 2005, 09:57:41 AM »

Tom are you using a step attenuator to determine dB? A receiver S meter is a bogus way to reference dB. But then I have a thing about bogus S meters
having a $100K R.S receiver to play with at work with a real accurate level meter.
(and a 2 inch speaker)
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« Reply #6 on: October 03, 2005, 11:05:52 AM »

Tom are you using a step attenuator to determine dB? A receiver S meter is a bogus way to reference dB. But then I have a thing about bogus S meters
having a $100K R.S receiver to play with at work with a real accurate level meter.
(and a 2 inch speaker)

The FT-1000D has a internal pads that I have used to verify the S meter. In addition, I've seen numerous guys do the 10X power linear in/out test to know the S meter is pretty close. I do have a calibrated step pad, but haven't had it in line for a while now.

Anyway, regardless of S meter calibration, the general diminished performance of the low dipole for CLOSE IN local signals surprised me. I figgered it wud be at least as good as the higher one for signals within 50 miles on 75M..  But for my line noise and dropping down unwanted DX signals, it's FB. OM.

T
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« Reply #7 on: October 03, 2005, 10:42:08 PM »

The efficiency of a dipole over ground is a function of it's height. The less conductive the ground, the greater the loss at low heights. As you can see in the plot below, once you get below 0.1 wavelength, things get pretty ugly. One-tenth of a wavelength on 75 meters is about 25 feet. The plot below was done at 18 MHz, so these numbers aren't directly comparable to 75 meters, but it's a safe bet the trend is the same.



Full sized plot at

http://www.amwindow.org/misc/gif/dipeffgnd.gif
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« Reply #8 on: October 03, 2005, 11:07:05 PM »

Interesting plot, Steve.

So, it appears that a dipole at .125 wavelengths high [~30' on 75M ] is about 85% efficient. That's not bad considering the ground used for that plot is one step above urban concrete...

I see the curve starts to flatten out anywhere above 40' high or so.

For RF, the average ground depth on 18mhz is a lot less than 75M, but I would agree that the trend is the same.

Based upon that plot, I would say that the differences I've been seeing between the low and high dipole are mostly angle of radiation... being that those angles above 75-90 degrees are kinda useless, even for close in.  An 85% eff antenna must be down no more than 0.3 db from a perfect one, I wud guess.

T
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« Reply #9 on: October 03, 2005, 11:22:23 PM »

Yep, 50% efficiency would only drop 3 dB. Another thing that may hurt low dipoles (probably not yours, since it's was away from everything) as that they will tend to couple RF into power lines, phone lines and other structures (thus telephone RFI and the like). In  my old neighborhood there were tons of wires, lines, CATV cable and chain line fences within a hundred foot radius of the antenna. The RF coupled into those structures isn't likely to make it to the ionosphere, so the signal suffers. RF getting into the telephone isn't getting into the receiver of the guy on the other end of the QSO.  Cry
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« Reply #10 on: October 04, 2005, 07:11:39 PM »

Steve said:
Quote
. RF getting into the telephone isn't getting into the receiver of the guy on the other end of the QSO

Now there's an astute observation!
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Steve - WB3HUZ
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« Reply #11 on: October 04, 2005, 08:34:36 PM »

Yea, I know it's stating the obvious. The idea was to get people to think about the issue. So many of us sort of pass off RFI sometimes, myself included. Sometimes you have to, there's nothing you can do about it. The point was, there are many things on or close to the ground that can negatively interact with a low antenna. Telephone RFI is just one easily notice symptom.
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« Reply #12 on: April 13, 2006, 07:16:55 PM »

not a bad test

actually the low slung dipole is working like it ashould

that is hsing nvis theory - near vertical incident skywave.

one thing u didnt consider in the test is the s/n ratio of the 2 antennas,  the 20-ft'er has a loweer noise floor relative to the signal being heard whereas the 70-ft has a low s/n cause of the power lines

also for it to work effieciently, BOTH stations need to have low antennas.  thats the resrarch i've been reading, as i have a 40m dipole streched at 7 ft off the ground using nvis technique.
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« Reply #13 on: April 13, 2006, 07:51:57 PM »

not a bad test
one thing u didnt consider in the test is the s/n ratio of the 2 antennas,  the 20-ft'er has a loweer noise floor relative to the signal being heard whereas the 70-ft has a low s/n cause of the power lines

Yes, this is true.   

But in this case, the s/n did not come into play.  I was comparing raw signal strength between the two using big receive signals. The signals coming in were usually S9+40 over at night, etc, totally masking the noise floor on either antenna which was below S9.

Threse days, the only time I use the lower dipole is during the day when signals are very weak and the high dipole has S9 line noise. But at night when the band noise masks the line noise, the high dipole is the winner for most paths.  As you suggested, the s/n advantage of the low dipole makes a big difference in the day.

Interesting on matching the two low dipoles and your 7' high 40M ant.

I notice this effect on 75M into Eu, but with high Yagis, low angles. I run a 2el Quad at 190' to Eu. When someone over there gets on with a high Yagi with a low angle, it makes a huge difference compared to a lower dipole. Much more than textbook. It's like the two angles get matched.  I saw a station from Ireland pin my S meter at S9+60 back in Dec, 1987. EI8H, Pat. He ran a 3el Yagi at 130'.  The lower dipoles never come within 20 db of that.


Angle of radiation is the most important factor in propagation and antenna performance. It can add or subtract  20 db at times, whereas forward gain can give maybe  4 to 6 db at best with modest Yagis.

73,
T

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« Reply #14 on: April 14, 2006, 01:24:14 PM »

Tom,
consider the close spaced beam. I bet if you lay out some ground wire under the ant things will change. BTW just built a quite accurate RA6830 DF  radio with calibrated step attenuator AGC. The SDR board will hang off the IF so i will be able to monitor signals within a DB of accurcy on the display. gfz
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« Reply #15 on: April 14, 2006, 01:40:53 PM »

Tom,
consider the close spaced beam. I bet if you lay out some ground wire under the ant things will change. BTW just built a quite accurate RA6830 DF  radio with calibrated step attenuator AGC. The SDR board will hang off the IF so i will be able to monitor signals within a DB of accurcy on the display. gfz

Frank,

Well, the thing is, I don't believe the angles above 80 degrees are worth a darn for anything but out 20 miles or so on 75M. Remeber that driven Zenith beam you saw here that had on  element at 150' and another at 60'?  Or the two stacked rotary dipoles I had at 190' and 80' fed 180 degrees out of phase?  Modeling showed them both to have big energy peaking at 80-90 degrees. But both were down or the same as a simple dipole at 60' high for local work.   I could not see any difference even for signals 30 miles away. There should have been a 6db improvement according to the modeling at zenith.

My point is that for 75M "local" work from 20-300 miles, you can't beat a dipole at 60'-80' high. The high zenith angles are useless for this path from what I've found.  70 degrees and lower is where you need it.  60 degrees may be perfect for local work on average.

But, during the middle day, the lowest angles work the best. I can see Tron up 10db on the high Quads compared to the dipole. But this edge disapears as late afternoon arrives.

Maybe it's different on other frequencies near the ocean for military high angle real close in coverage, but for ham radio 75M work, I found I wasted a lot of time trying > 80 degree arrays.

73,
T
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« Reply #16 on: April 14, 2006, 04:18:46 PM »

Hi Tom and all,

I use EZNEC 3.0 for antenna simulations here for many years now.  The program is written by Roy Lewallen W7EL, copyright 2001. 

It does a great job of modeling antennas close to the ground and you can select real ground and an optional high-accuracy real ground.  You can provide your own numbers for the soil conductivity and its dielectric constant also.  You can choose real antenna conductor loss – copper, aluminum, zinc, tin or zero loss.  The older programs usually model the radiation patterns well, but the absolute gain and feed point impedance is not accurate.  I do have a very good confidence in the accuracy of EZNEC 3.0 .

When an antenna is close to the ground like a 30 foot high dipole at 75 meters, the pattern does peak at 90 degrees elevation, but the antenna is lossier due to its proximity to ground.  So it can be a little misleading to look at pattern only, you need to compare the absolute gain of the 30’ dipole and 70’ dipole at the various radiation angles and azimuths.  The two opposing factors of pattern and absolute gain tend to cancel out in this situation at the very high elevation angles.

According to Tom’s post I picked 4 distances – 70, 210, 350, and 485 miles (multiples of 1 degree of latitude change). 

Using the Ham CAP 1.4 ionospheric prediction program I came up with these elevation angles for 75 meters for April, 1 p.m. and 5 p.m. in the northeastern U.S.:

                1 p.m.     5 p.m.
70 miles     66 d.      78 degrees
210 miles   30 d.      54 d.       
350 miles   18 d.      18 d.
485 miles   12 d.      12 d.

EZNEC elevation angle dBi results:
    dipole @ 30 ft.
          0 d. azimuth       90 d. azimuth
12 d.       -6.11 dBi         -11.44 dBi
18 d.       -2.9 dBi             -9.07 dBi
30 d.        0.77 dBi           - 5.1 dBi
54 d.        3.99 dBi              1.43 dBi
66 d.        4.67 dBi              3.77 dBi
78 d.        5.02 dBi              4.73 dBi

    dipole @ 70 ft.
          0 d. azimuth       90 d. azimuth
12 d.       -1.64 dBi         -12.99 dBi
18 d.        1.36 dBi           -9.83 dBi
30 d.        4.41 dBi           - 4.13 dBi
54 d.        5.87 dBi              2.75 dBi
66 d.        5.70 dBi              4.31 dBi
78 d.        5.44 dBi              5.10 dBi

Let’s see how the EZNEC results compare to Tom’s over-the-air results:
First of all, at high angles the dipole’s orientation is not a factor since the azimuth pattern is almost perfectly omni-directional, but at low angles the pattern is quite directive and the differences in azimuth can result in over 11 dB. worst case.

Just looking at the difference between the 2 dipoles at 78 degrees elevation angle, the 70 ft. high dipole has 0.4 dB better absolute gain than the 30 ft. high dipole whether broadside or off the end of the antenna.

At 30 degrees elevation angle, the 70 ft. high dipole has 1 to 3.6 dB. better absolute gain than the 30 ft. high dipole, depending on signal azimuth.

At 12 degrees elevation angle, the 70 ft. high dipole has 4.5 dB. better to 1.6 dB worse absolute gain than the 30 ft. high dipole, depending on signal azimuth.  At these low elevation angles the antenna azimuth is as important as the height.  People with the 30 ft. high antenna can do as well as an adversely-oriented 70 ft. high dipole. 

For those who need to pecker-match working Dean-o Klay-N-neX, orient your dipole wires north-south!

All-in-all I would say that the there is no advantage to the 30 ft. high dipole as compared to the 70 ft. high dipole by the computer analysis with EZNEC, as Tom found out in his on-the-air tests. 

Well, this is what my computer programs came up with.  I leave further analysis of the results table to you.
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« Reply #17 on: April 14, 2006, 06:00:06 PM »

Vely vely interesting, Tom!

It sure shows the uselessness of using a high angle or zenith radiator for 75M.

What stands out to me is the optimum angle numbers you found for 1PM and 5PM on 75 M below:

----------------------
Using the Ham CAP 1.4 ionospheric prediction program I came up with these elevation angles for 75 meters for April, 1 p.m. and 5 p.m. in the northeastern U.S.:

                1 p.m.     5 p.m.
70 miles     66 d.      78 degrees
210 miles   30 d.      54 d.       
350 miles   18 d.      18 d.
485 miles   12 d.      12 d.
---------------------------------------


It's interesting that at 350 miles and greater, the angles are so low!  I wonder if that's because the time is still too early, before sunset. Could you run that again for  sunset, 1 hour after and then 5 hours after?

In the real world, I do notice a big advantage of lower angles in the day. But at night, these low angle antennas are noticeably down locally unless the stations are out farther than 500 miles++, etc.

At midnight, stations have to be at least 800 miles away and more for the low angles to have the edge.

Bottom line is that to cover most vertical take-off angle situations, we need two antennas - one at 1/4 wave high and one at 1/2 wave high.

T
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« Reply #18 on: April 14, 2006, 09:57:10 PM »

O.k. Tom,

Here are more details from the Ham CAP program:

The killer in the daytime contacts is that the skip is via the E layer.

Note: All of these contacts are single hop.
                                            Sunset   sunset+1   sunset+5
                1 p.m.     5 p.m.    7:30 p.   8:30 p.    12:30 a. EDT
70 miles     66 d.      78 d.      77 d.      78 d.        82 degrees
Layer          E             F1         F2         F2             F2
MUF         5.8 MHz.  5.9 Mc    5.8 Mc    5.2 Mc      3.3 MHz.

210 miles   30 d.      54 d.       55 d.     56 d.        65 degrees
Layer          E            F1           F2         F2            F2
MUF        6.0 Mc.    6.2 Mc.    6.2 Mc.  5.6 Mc.      3.4 MHz.

350 miles   18 d.      18 d.       40 d.      41 d.       52 degrees
Layer           E           E             F2         F2           F2
MUF        7.6 Mc.    7.1 Mc.   7.1 Mc.    6.4 Mc.     4.0 MHz.

485 miles   12 d.      12 d.        31 d.     30 d.       37 degrees
Layer            E           E             F2         F2           F2
MUF         9.9 Mc.   8.1 Mc.     8.3 Mc.  7.3 Mc.     4.4 MHz.
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« Reply #19 on: May 21, 2012, 02:20:04 PM »

Tom, just wondering if you still have the two dipoles up?  If so, I would be interested in hearing how they compare off the ends of the dipole at the different distances. 

Al, n7ioh
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« Reply #20 on: May 21, 2012, 03:04:11 PM »

Talk about a blast from the past!

I'm sure Tom will have plenty to say.
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« Reply #21 on: May 21, 2012, 03:25:11 PM »

Tom,

You have effectively described the advantage of a NVIS antenna.  It takes a small 2 dB hit on transmit and receive, but the distant qrm is reduced by 6 to 9 dB.  Just what you want for close-in work.  If you want DX, of course make it as high as possible.  Good test work, thanks for sharing.

Best regards,
Chris
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« Reply #22 on: May 21, 2012, 06:14:32 PM »

Tom,

You have effectively described the advantage of a NVIS antenna.  It takes a small 2 dB hit on transmit and receive, but the distant qrm is reduced by 6 to 9 dB.  Just what you want for close-in work.  If you want DX, of course make it as high as possible.  Good test work, thanks for sharing.

Best regards,
Chris


Oh, but the unlimited time and energy I tinkled away on antennas in years gone by...    :-)

Chris, you bring up a good point.  Using a low dipole to cut back the distant QRM is something I forgot about. I remember seeing its effect when the louder ssb stations started coming in and  taking out the local AMers on the high dipole - but making it easy copy on the low dipole.  


Since that time I turned the low 75M dipole into a 40M dipole for local work.   But in the wintertime, that low 75M dipole would be an excellent antenna for the high angle locals - kinda like an antenna QRM filter.

T

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