Effects of height on dipole

[distortion]

I am about to do some testing thursday with my dipole at very large heights. I am curious at what point will there cease to be any gains in the antenna that I have.

I am planning on putting this thing up at least 50 feet up. I am curious as to how height effects a dipole? Does it give any noticable gain. It will be in the inverted v format if at all possible framed with balsa wood.

I am curious.. will this antenna just work better and better the higher it gets? Or will at some point like, say, 100-200 feet, will the antenna start to lose effectiveness?

Thanks,
Josh

 

[W5LZ]

Josh,
While antenna height does affect a number of things the two that are most affected are the antenna's input impedance and the shape of it's radiation pattern. It's gain isn't affected much at all, except in what directions it has gain in, not the total.
An antenna's input impedance is affected in a variable way (if that makes sense). Meaning it will vary around the 'ideal' 50 ohms, going above and then below, gradually damping out till it settles on something near about 75 ohms (those values are not exact by any means and vary according to actual antenna height and it's surroundings).
Most antennas are not very 'directional' when under about 1/2-1 wavelength in height above ground. Dipoles start in an omnidirectional pattern near ground, gradually change to something in the shape of a football, then into the 'classic' figure 8 sort of radiation pattern. Don't let the usual thought that this radiation pattern is really a 'figure-8', it really isn't, exactly. As in the 'nulls' off the ends are never super sharp, you never loose signals off the ends completely but they are reduced quite a bit. (Most people know what an "8" looks like and it's a good ~'general'~ description. Not cast in stone!) The
radiation pattern is affected by almost everything around the antenna, how the antenna is constructed, it's 'shape' (flat/'V'/whatever). Unfortunately, it ain't never exactly 'pure' and what you may think it is, sort of. In most cases it doesn't make enough difference to make any difference. Even the amount of sap in the surrounding trees will make a small difference.
In general (meaning there are always exceptions), the effectiveness of an antenna increases with height till you get to some point where feed line losses, and COST, out weight the whatever the increase effectiveness gains. Remember that 'height' is in relation to wavelength, not just feet/yards/miles. There comes a point where being 'practical' enter$ the equation. Then it's time to do something else.
None of all that takes propagation into account. Sometimes lower would be 'better'. There's no really sure way of predicting that so "higher is better" is true on the average. Unless you have more money than sense and make your antennas variable in height, sort of a vertical 'rotor' do-hicky?
The only absolutely true statement about antennas is TANSTAAFL! (There Ain't No Such Thing As A Free Lunch)...
- 'Doc

PS - Go for it!

 

[Hamin' X]

Well said, Doc. The only thing that I would add is to beware of feedline radiation, distorting the pattern. A dipole is a balanced antenna and most people feed it with coax (Unbalanced feedline). Use a balun, or at the least a coil of the coax at the feedpoint, to choke off the unwanted feedline radiation.

Rich

 

[freecell]

for DX work, higher placement is warranted, since more power concentrated between 5 and 15 degrees is reported to be of major benefit. heights around one wavelength are necessary to get the broadside lobe to launch in this range. however, higher may not always be better. pay careful attention to the magnitude of secondary lobes in the broadside direction, as well as high angle radiation off the ends. some heights would appear better than others due to concerns with nulling out local QRM.

for local work, lower heights appear to be more beneficial. note especially how omni-directional the dipole becomes at lower heights. below 0.4 wavelengths, there is less than 1 dB of attenuation in the end fire direction, which suggests a height between 0.4 and 0.3 might be an ideal compromise for local nets and rag chewing.

 

[Master Chief]

Freecell, you forgot to give credit to the author:

http://www.qsl.net/aa3rl/ant2.html

Look for, "So, How High should the dipole be to work well?"

 

[freecell]

no, he forgot to credit the author....the ARRL Handbook. i'm just freely distributing with permission of the webmaster.

 

[Master Chief]

These are Mike's words (word for word), and not the ARRL's. Ideas and such may be similar, but Mike wrote the article.

Mike says to distribute freely, but you should still give him credit! Otherwise, people here my mistakenly think that you wrote this, and that wouldn't be fair to the author.

 

[freecell]

no worry there, i have you for that....the original poster didn't ask about any author and i don't really care.

i plugged the appropriate search terms into a computer and forwarded the information constituting the answer to his question. if anyone really cares who the author is they can follow the same procedure for themselves.

http://forum.worldwidedx.com/viewtopic.php?p=57377&highlight=copy+paste#57377

 

[distortion]

So coil the coax towards the antenna side of the coax, or towards the radio side?

I am actually doing an experiment thursday.. I am going to hook a di-pole up to a a rather large weather balloon (or 2), and build an inverted V frame to put the dipole in, and I'm going to slowly raise the antenna 10 ft or so at a time, and see what kind of difference in reports I get from a local, distance, and if skip is in there (which it usually is around that time). I have 100 ft of coax I can use. I'm thinking that it'll be interesting to see the results, based upon what the articles say and comparing it to real world experience.

Any idea which form of the dipole would give the best radiation pattern for local talking with 100 ft of elevation of the radiowavz dipole? I'm considering trying just the vertical setup, and the inverted v.. but I have all day to mess with it, so any suggestions would be appreciated.

Thanks for all your replies, and for some interesting posts!!!

Thanks,
Josh

 

[Master Chief]

no worry there, i have you for that

Glad I can help shed some light on things.

the original poster didn't ask about any author and i don't really care.

i plugged the appropriate search terms into a computer and forwarded the information constituting the answer to his question. if anyone really cares who the author is they can follow the same procedure for themselves.

http://forum.worldwidedx.com/viewtopic.php?p=57377&highlight=copy+paste#57377

Interesting reading. In the above text, you did take the time to change the punctuation from capitals to lower case. Why not just add the quotation marks at the beginning and end. That would be MUCH easier! Or better yet, I like your idea of just including the link! This gives proper credit (whether you care or not) and is the easiest of all. If you are going to take the time to look it up for them, why not take the extra second to do it right.

I agree with you 100% that a simple search in Google will ferret out the answers to their own questions.

 

[freecell]

you bet, Josh....

heights around one wavelength are required to get the broadside lobe to launch in the range of possible take off angles between 5 and 15 degrees, since more power concentrated at these angles is known to be an operating plus when working DX. but remember, higher is not always be better. pay careful attention to the magnitude and multiplicity of secondary lobes in the broadside direction, as well as high angle radiation off the ends, especially at heights exceedingly above 1 wavelength. you will find that some heights perform better than others with regard to nulling out local QRM.

for local work, lower heights below 1/2 wavelength perform well. the dipole becomes much more omni-directional at these lower heights. there is less than 1 dB of attenuation in the end fire direction below 0.4 wavelengths, which clearly indicates that a height between 0.4 and 0.3 might be more desirable for local nets, rag chewing and NVIS operation when conditions permit, relative to the frequency in use.

the only reason i can see for heights in excess of 1 wavelength is to clear local terrain for improved ground wave or direct, point to-point communications or to distribute the power generated across a wide range of take off angles through the upper 180 degree arc of the vertical axis for whatever purpose/s. above 1 wavelength and further the high angle radiation including multiple high angle lobes become more prevalent and of virtually no use for local operation without f1/f2 layer activity present, while attenuating the gain at any specific or normally advantageous angles required for low angle DX. if you want good local coverage then 1/2 wavelength and below will give it to you. if you want primarily DX then 1/2 wavelength to 1 wavelength above ground will fill the bill.

you can keep your experimentation at or below 1 wavelength above ground to the feedpoint and use the dipole for both local and dx work. the ultimate would be to be able to change the feedpoint height from 1/2 wavelength (or less) to 1 wavelength (or anywhere in between) at will to avail yourself of the best of both worlds. the inverted v can be constructed of aluminum tubing in the same fashion as the dipole so you can have some fun with that as well.

you can do all of that and you're 13 feet closer to the ground than you were planning on. work smarter, not harder.

now all you have to decide is how you want to use it.

have fun.

 

[W5LZ]

Josh,
If you decide to use a 'choke', put it at the feed point of the antenna, the same place you'd put a balun. It doesn't really matter till you get to the point where things change from ballanced to unballanced, right?
Or, don't use one at all. The ballanced to unballanced thingy just isn't that detrimental at HF. It does cause a sort of 'slewing' of the radiation pattern but it isn't very pronounced, and unnoticable in almost every case. Oh, also don't wind the thingy too tight, or in very small coils. All that does is cause distortion in the coax (migration). I'd say try it with/without the choke but that's a lot of bother... Ah, try it anyway. Tell me how much difference you hear.
- 'Doc