On 3 abr, 00:42, "Richard Fry" wrote:
Followup -- the link below compares the relative current distribution,
directivity and radiation efficiency of a helical and a linear radiator
system when the helical radiator described in my earlier post is operating
at the frequency of its first self-resonance, and the linear monopole height
is set for its first self-resonance at that same frequency.

It is interesting to note that linear form has better performance than the
helical form.

http://i62.photobucket.com/albums/h8...inear_1st_Reso...

Hello Richard Fry,

Why is this so interesting, as it is what I expect (and I think you
expect this also)? The current*(physical length) product is more, so
given same feed current it produces more field (hence more radiated
power). This results in higher input impedance, hence reducing the 10
ohms ground loss.

The small change in shape of pattern is just due to the less isotropic
array pattern of the 0.25 lambda radiator (w.r.t. to the array pattern
of the 3 m radiator).

If it is not time consuming, I would like to see what happens when you
extend the helix until it gets its second (half wave) high impedance
resonance (current maximum in the middle). I expect some gain increase
due to small change in antenna pattern and reduced ground loss.

¡Very informative thread!

73, Wim, PA3DJS, www.tetech.nl.

On Apr 2, 7:33*pm, Wimpie wrote:
I would like to see what happens when you
extend the helix until it gets its second (half wave) high impedance
resonance (current maximum in the middle).

Also at its third (3/4WL) low impedance resonance where there are two
current maximum points on the helix.
--
73, Cecil, w5dxp.com

"Wimpie" wrote
If it is not time consuming, I would like to see what happens when you
extend the helix until it gets its second (half wave) high impedance
resonance (current maximum in the middle). I expect some gain increase
due to small change in antenna pattern and reduced ground loss.

The NEC software that I used for the results shown earlier doesn't allow
enough segments to double the length of that helix. So I reconstructed the
model in one that does -- link to screen clip from it below:

http://i62.photobucket.com/albums/h8...onopole_6m.gif

On 3 abr, 21:11, "Richard Fry" wrote:
"Wimpie" wrote

If it is not time consuming, I would like to see what happens when you
extend the helix until it gets its second (half wave) high impedance
resonance (current maximum in the middle). I expect some gain increase
due to small change in antenna pattern and reduced ground loss.

The NEC software that I used for the results shown earlier doesn't allow
enough segments to double the length of that helix. *So I reconstructed the
model in one that does -- link to screen clip from it below:

http://i62.photobucket.com/albums/h8...onopole_6m.gif

Hello Richard Fry,

Thanks for taking the time to do the simulation.

It seems from your graph that the electrically half wave (180 degr)
helical has slightly more gain than the physically and electrically
quarter wave (90 degr) radiator (5.61 dBi for the helical versus 5.14
dBi for the quarter wave whip).

Given the about 4 kOhms for 1/(Re(Y)), the loss in the 10 Ohms ground
resistance is negligible (so at can be significantly higher before
presenting significant loss). 900Vp at the feed point (100W input)
will not give problems with matching.

With kind regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely

On Apr 3, 3:42*pm, Wimpie wrote:
It seems from your graph that the electrically half wave (180 degr)
helical has slightly more gain than the physically and electrically
quarter wave (90 degr) radiator (5.61 dBi for the helical versus 5.14
dBi for the quarter wave whip).

A full-length half-wave monopole has gain over a full-length 1/4WL
monopole because the current maximum point on the half-wave is
elevated by 1/4WL. The same concept has to be a technical fact for the
helical even though muted by the smaller physical distance between the
current maximum points.
--
73, Cecil, w5dxp.com