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Author Topic: DiPole Height  (Read 2728 times)
N6SBN
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« on: June 27, 2013, 02:17:04 PM »


  Alpha Delta DX-ee.    The center is up 29'.  The ends of the dipole drop to about 10' off the ground.  For this antenna to work well, do the ends need to be up at 30' and the center ? 45' . 

  Listening to the radio, I get the feeling that the antenna signal is more vertical.  I'm getting different, closer in contacts.    I'm comparing this to my previous antenna,  a 31' vertical that was mounted up on the roof with nine 31' radials.
 
   Thanks in advance.
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WB6BYU
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« Reply #1 on: June 27, 2013, 02:33:16 PM »

What band are you using?

The vertical angle of radiation is important for determining the signal strength vs. distance.
With a horizontal dipole, that is dependent on the height above ground.  The higher the
antenna, the more radiation at low angles.  Ideally for longer distances you would want
to get the antenna up 1/2 wavelength or more.

On 40m, 29' is quite low, and maximum radiation will be straight up.  On 15m it is about
5/8 wavelength and will work much better for DX.  That's why knowing the frequency
of interest is important.

By comparison a vertical has a null overhead in the pattern:  it won't pick up local stations
as well (assuming they are out of range for ground wave).  Whether it is better for DX will
depend on the ground conditions and the height of the dipole you are comparing it to.
Generally the height at which a horizontal antenna works better for DX than a vertical is
somewhere around 1/4 to 3/8 wavelength, but depends on a lot of factors.  That's why
verticals are often used for DX on 40m, 80m and 160m, but a dipole may be better on
20m and higher frequencies because it more practical to get enough height (in wavelengths.)

Getting the ends of the antenna up higher will help.  Raising the whole antenna will help
even more.  But there is no set height at which an antenna starts working DX:  it is a
continuous function of height.
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K0ZN
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« Reply #2 on: June 27, 2013, 03:31:42 PM »

Your observation is correct. That antenna is a very high angle radiator and would do a good job for close in skip contacts. It would be poor for a DX antenna,
especially on the lower bands.  A vertical (with a good radial system or ground plane) is a very good LOW angle radiator, therefore "usually" better for DX than
a low horizontal antenna.  A high horizontal antenna is another game entirely and can be quite effective for long haul DX. The old saying that higher is better
is true in most cases at the heights the average ham can manage.

The following comments are meant to be a help and not a criticism. I think you would find it very beneficial and informative to look at the ARRL Antenna Book (an older one off ebay is fine...) and check out how radiation patterns change with height, polarization and length. The subject is way too complex for a few paragraphs on here. The graphics and details in a real antenna book are extremely beneficial info for a ham to understand when picking antennas, etc.  Antenna knowledge can save you a lot of wondering and frustration.


73,  K0ZN

« Last Edit: June 27, 2013, 03:34:24 PM by K0ZN » Logged
N4JTE
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« Reply #3 on: June 27, 2013, 06:07:55 PM »

To achieve the best efficency/contacts beyond a few miles the whole antenna needs to be as high as possible, if your situation does not allow that, then at least try to get it more flattopped and raise the ends as high as possible.
Bob
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KB6HRT
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« Reply #4 on: July 02, 2013, 01:33:08 PM »

Been in radio a while now and the proper height of a dipole is the height that get the job done most of the time for your needs
that is the correct height, for me that about 36' above the ground because am a Rag Chewer on 75 meters in the mornings, talk to stations from 1-1500 miles away most every morning for the last 15+ years an do it 350+ days a year. If I were a DXer that maybe different, but do listen other times in the day but don't transmit, how did I come up with 36' well if I went higher my antenna brought in more noise also more signals but had to pull them out of more noise. every HAM station will be a little different
because there are so many variables one has to contend with, so a good rule of thumb is I start at 36' and make the antenna mast so you can go up or down about 5' feet higher (more signal) lower (better signal to noise ratio), you can work out, over time what works best for your setup. If you change your setup an add an AMP or tuner or new radio or coax things may change. I have two AMPS an when I change from one to the other signal changes to my radio, by ear can tell which AMP  have in line. The closer your antenna is to 1 to 1 the more noise the radio brings in so most of the time on 40-75-80m I use a G5RV for Rag Chewing because it works well and the nature of the G5RV antenna is it brings in less noise but still brings in most all the signals well with a good signal to noise ratio an it will talk that 1500 mile circle I was talking about......73s.................kb6hrt
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FOXBAT426
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« Reply #5 on: July 02, 2013, 06:09:00 PM »

Good info!!
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N6SBN
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« Reply #6 on: July 03, 2013, 08:56:18 PM »

I think the answer to my problem is to get a crank up  mast.  Raise it for DX and lower it for local rag chew.  The ends could be on pullys to allow the ends to adjust.  Craig N6SBN
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RFRY
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« Reply #7 on: July 04, 2013, 03:59:50 AM »

The (practical) height of a horizontal dipole above the earth has relatively less affect on its radiation pattern on the lower HF bands.

The link below compares the far-field radiation pattern of the same dipole on the same frequency for 5 mS/m earth conductivity, and heights of 12m and 18m.  Their patterns are quite similar.

http://s20.postimg.org/v8e1wks8d/75m_H_Dipoles_Compared.jpg
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KC4MOP
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« Reply #8 on: July 04, 2013, 04:36:21 AM »

I think the answer to my problem is to get a crank up  mast.  Raise it for DX and lower it for local rag chew.  The ends could be on pullys to allow the ends to adjust.  Craig N6SBN
OR you build two tall towers about 100 feet high and have several dipoles mounted on them. And switch for the dipole you want for local 200-500 mi or DX. The dipole needs to be 1/2 wavelength above ground to be DX. To be practical 40M on up can be workable. 1/2 wavelength for 40M is about 65 feet. 30 feet high is a "local" antenna.
An easy way for DX is to build a vertical antenna.
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WX7G
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« Reply #9 on: July 04, 2013, 06:35:13 AM »

Let's quantify this. Based on the other antenna, a 31' vertical, I assume the frequency of interest is 7 MHz.

Comparing the DX-EE with the center at 29' vs 45' (the ends are 14' lower) for various take-off-angles and broadside to the antenna the higher antenna gain advantage is:

  5 degrees, 3.4 dB
10 degrees, 3.3 dB
20 degrees, 3.0 dB
30 degrees, 2.6 dB
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KB4QAA
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« Reply #10 on: July 04, 2013, 12:37:40 PM »

With the peak at 29' and the ends at 10ft, the effective height is well less than 20 feet, maybe close to 15ft.   Draw it on graph paper and see for yourself.

Below about 1/2wl, dipole antennas become essentially omnidirectional with high angles.  Additionally, the lower the height, the more coupling to earth with loss of radiated power, and changes to impedance.

That said, you can still make lots of contacts, even DX with the right conditions and have lots of fun!

The single most important thing you can do to improve your performance is to elevate the ends and get close to a level condition. (catenary or droop is not critical).  bill
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WH7DX
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« Reply #11 on: July 04, 2013, 01:06:00 PM »

I think the answer to my problem is to get a crank up  mast.  Raise it for DX and lower it for local rag chew.  The ends could be on pullys to allow the ends to adjust.  Craig N6SBN

Get a push up mast for less than $200.   You might find one for $100 or so.   Simple.   Pipe in the ground.. Pushup over it.   

I have a low 80M and 160M (like 30ft at center and 10ft at ends) and get out 5,000 miles.   Higher would be better - but it's what I have.

You could do 3 push up masts.   Get the Center up 30-35ft and the ends up 20ft?

http://www.3starinc.com/40_foot_telescopic_push-up_antenna_mast_ups_shippable.html
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WX7G
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« Reply #12 on: July 05, 2013, 09:16:15 AM »

Is a tower for this antenna worth 3 dB or would the money be better spent on an amplifier. An AL-811 amp provides 8 dB (over 100 watts) for $700.
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WH7DX
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« Reply #13 on: July 05, 2013, 01:41:44 PM »

Roger...   get a cheap pushup mast to get the dipole up as high as needed and then get a nice amp like the 811H for $900.    I have had no problems with that amp and it works with my current power supply config.   Just paste the ANT settings on the front and dial it in.

Going from 80W to 700W is about 9.4 dB gain and I see about 2  S Unit increase I believe.  So for those tough ones.. $900 and regular electrical is certainly worth the price for 2 S units.   Hex Beam adds another 1-2 Units as it's directional and LMR-400 will last for decades and loses basically nothing.
« Last Edit: July 05, 2013, 02:44:33 PM by WH7DX » Logged
KH2G
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« Reply #14 on: July 06, 2013, 05:58:13 PM »

A comment was made to the effect that you could run an amp and be done with it and the problem there is that if you can't hear them, you can't work them.  The type soil you have makes a difference also and bottom line is as has been stated - the higher and flatter you can get it, the better off you are but then I ask = What do you want your antenna to do DX or just local rag chew? Enjoying the hobby is bottom line.
73, Enjoy
Dick
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