Really any difference at height?

[wavrider]

http://www.hamuniverse.com/wb4yjtdipolepatterns.html

I have referenced this article several times it is a good read , notice the radiation pattern at 1/2 WL AGL,

 

[Beetle]

How, and how high, to put it up depends on you. Try different configurations at different heights and find what works best in YOUR location. Don't overthink things.

 

[RadioDaze]

With regards to a 5/8 wave with no radials. (we commonly see "half an antenna" written after this.) If end fed designs are susceptible to common mode currents on coax/metal masts (especially 5/8 waves) and you do not have radials, but you choke effectively and isolate your mast where do these currents dissipate and what are the effects?

 

[RadioDaze]

http://www.hamuniverse.com/wb4yjtdipolepatterns.html

I have referenced this article several times it is a good read , notice the radiation pattern at 1/2 WL AGL,

What I found confusing about that article when I stumbled on it in the past was whether these were vertical or horizontal plots. He speaks about horizontal dipoles in the first paragraph so you would assume horizontal. But the plots, I cannot work out if they are horizontal or vertical polarized antennas, that is quite confusing. We should not over think but nor under think.

He still says these are horizontal plots. We are talking about verticals.They may not be relevant in direct comparison. Looks like some nasty nulls in those plots 1.25 WL and above.

Whereas the lower height ones are wider despite maybe 7-10dB less gain at the lower angles.

Looking at those plots for DX alone, 0.5 WL seems to be the best compromise to suit your suggestion. (probably for F2 and Sporadic E skip) It has that broad wide lobe despite the lower gain.

I would have thought in this situation with a basic dipole it might be a good case for increasing power (to increase gain) within reason and using a lower antenna.

Though again 5/8th waves are typically vertical.

 

[bob85]

w8ji & kirchhoff's current law tell us you can't isolate the antenna from any conductive mast and effectively choke the coax braid to minimise common mode unless you give the currents somewhere else to flow such as radials.

 

[The DB]

Yet I've seen this setup work very well multiple times, at least on the a99/imax antennas. The reason is where the coax plugs in to the antenna is not actually the antenna's feedpoint. In fact said feedpoint is actually a ways up inside that aluminum tube used to mount the antenna. On these antennas, that (and perhaps the aluminum tube itself) is more than enough to handle the remaining currents in play. Intentionally or otherwise, the design of these antennas takes into account the needs of Kirchhoff's current law. They don't violate W8LI's statements either.


The DB

 

[Marconi]

Daze, as you suggest the patterns in the link above look to me to be results from horizontal antennas.

 

[RadioDaze]

I did notice that on the IMAX insides there is a bit of coax that threads through to the dual coil/capacitor(brass tube) matching network. I considered this when I was planning radials. More about that on another thread. (It did not go very well)

 

[bob85]

DB have you ever modeled the imax including the coax and tube with no feedline and the feedpoint where it actually is on the imax ?

 

[wavrider]

is their any real benifts if put an IMAX 2000 or even a dipole up at 36' vs 45' vs 55 or 65'?

First post mentions dipoles at height?????

Radio Daze quote"He still says these are horizontal plots. We are talking about verticals."

 

[Heavy Metal]

So should I attach the antenna directly to the pole or isolate it?

 

[Marconi]

DB have you ever modeled the imax including the coax and tube with no feedline and the feedpoint where it actually is on the imax ?

Bob does the schematic below show the feed point location you noted?

If so, I'm curious if you would show me specifically where you see the feed point...so I won't have to guess your meaning?

http://www.copperelectronics.com/discus4/messages/7750/10779.gif

 

[The DB]

DB have you ever modeled the imax including the coax and tube with no feedline and the feedpoint where it actually is on the imax ?

I have. Although I have done more with the a99 than the imax, but I have done both. Did you have a question about them?


The DB

 

[bob85]

It would be interesting to see what NEC shows for current distribution on the short coax and aluminum tube below the feedpoint.

 

[The DB]

Here are two pics for you.

The first picture is simulating the length of said tube on the imax and the second is using a much shorter lower tube than the imax. As you can see, in both cases, the current on said element drops to nothing at the tip.

The shorter element has more low angle gain. Its not much in this case, but it is there. If you lengthen the lower tube or use a coax to simulate a choke 1/4 wavelength below the antenna as many people recommend for this antenna this gets much worse...


The DB