Marconi's new Gain Master with a matching network

[Marconi]

Here is my new GM model, but the addition of a capacitor in the middle of the antenna does not benefit the model with increased gain like we see in The DB's Imax model with a 42Pf capacitor in the middle of the radiator. The gain for the GM antenna with the capacitor is just a tad better than my model without the capacitor.

The match looks really nice and the SWR bandwidth at 3.20 MHz is a bit short of what specs call for at 4.50 MHz, vs. the 5.00+ MHz I get with my Real World antenna using 100' feet of LMR400 for the feed line.

 

[Needle Bender]

The 5/8 GM already uses phase correction to achieve the full 5/8 in-phase current shown in the CST, or am I missing something?

...a thing of beauty.

 

[bob85]

Must be missing something.
Any dipole up to 2 x 1/2 wave has all currents in phase. Theres no need for any phase correction.

Have a look at the x-pole capacitive top coupled j-pole.
It has a similar double dip wide vswr curve as the gain-master.

 

[Needle Bender]

Must be missing something.
Any dipole up to 2 x 1/2 wave has all currents in phase. Theres no need for any phase correction.

Have a look at the x-pole capacitive top coupled j-pole.
It has a similar double dip wide vswr curve as the gain-master.

Huh?
Unless I'm misreading your post, Bob, you're saying a center-fed full wave dipole with no phase correcting network has all currents in-phase?

 

[bob85]

NB
Yes that's what i mean.

Can you explain why you think the gainmaster has phase correction in order for the currents to be in phase over 5/8 of radiator.

All i see is an extended t2lt coaxial dipole using stub matching & capacitive coupling to match the higher feedpoint impedance.
and some fancy advertising.

downside of stub matching is vswr on the coax above the stub is always high reducing power rating for the coax.

Rg58 with high vswr won't take 500w fm.

Sirio could have made the gainmaster longer.

 

[Marconi]

NB, here is what Eznec predicts for a 2 x 1/2 wave, that I think is similar to what Bob has noted.


Here is the tabular currents log for this model and you will notice the only object besides the radiator is the feed point (the little red O)...no network, no capacitor, nothing else and I have the currents turned on in the antenna view also. We have a slightly unbalanced feed point.


It's too bad it has a terrible match that would need to be fixed.

 

[bob85]

That's ^ a little longer than resonant Eddie but it shows what im talking about.

If you extend each leg longer than 1/2wave you get out of phase radiation like any longer than 1/2wave end fed element.
increased physical spacing of the outer 1/2waves more than compensates up to 2 x .64wave.

After that you start thinking about phase correction.

 

[Needle Bender]

NB
Yes that's what i mean.

Can you explain why you think the gainmaster has phase correction in order for the currents to be in phase over 5/8 of radiator.

All i see is an extended t2lt coaxial dipole using stub matching & capacitive coupling to match the higher feedpoint impedance.
and some fancy advertising.

downside of stub matching is vswr on the coax above the stub is always high reducing power rating for the coax.

Rg58 with high vswr won't take 500w fm.

Sirio could have made the gainmaster longer.

OK, if we're talking about the 5/8 GM, I believe the phase correction realigns the current so the entire 5/8 radiating surface currents are constructive.

On a balanced dipole you can't go beyond 1/4 wave per side without phase correction or else you'll add unconstructive current.

...and I'd be surprised if you didn't already know this, so why are we having this discussion?

 

[bob85]

That's not how antennas work NB.

The example you posted is not a 2 x 1/2wave dipole.
A 2 x1/2 wave dipole has no deconstructive currents & provides about 2db gain over a single dipole

Gain increases as you increase length beyond 2 x1/2 wave up to ( 2 x.64 wave the highest gain vertical without phasing ).
any longer & the out of phase currents neutralise the benefit of increased spacing for the outer 1/2wave portions of radiator. high angle lobes start to dominate.
Just like a vertical groundplane.

Im equally surprised that you think anything over 1/4 wave per leg generates deconstructive currents or that there is phase correction needed on a 5/8 dipole.

 

[Needle Bender]

That's not how antennas work NB.

The example you posted is not a 2 x 1/2wave dipole.
A 2 x1/2 wave dipole has no deconstructive currents & provides about 2db gain over a single dipole

Gain increases as you increase length beyond 2 x1/2 wave up to ( 2 x.64 wave the highest gain vertical without phasing ).
any longer & the out of phase currents neutralise the benefit of increased spacing for the outer 1/2wave portions of radiator. high angle lobes start to dominate.
Just like a vertical groundplane.

Im equally surprised that you think anything over 1/4 wave per leg generates deconstructive currents or that there is phase correction needed on a 5/8 dipole.

OK, I guess if you like a broadside null & a 4-leaf clover pattern, but I'm still under the impression that .5 +.5 = 1
BTW, how do you like to feed that 2 x 1/2 wave, or 2 x .64, dipole?

 

[The DB]

OK, I guess if you like a broadside null & a 4-leaf clover pattern, but I'm still under the impression that .5 +.5 = 1

Its not the length of the antenna that matters, it is the length of the individual elements. If the elements are electrically 1/2 wavelength long, or shorter than, and you have one on either side of the center feed point, all of both sides will be in phase. It isn't until one or both elements are longer than 1/2 wavelength that you begin to have out of phase currents. The only reason that two 1/4 wavelength elements is commonly used is it is easy to match to a feed line with reasonable SWR. This type of setup (the full wavelength antenna) will require some tuning, and when used on ham radio will most often use parallel feed line running to an antenna tuner for said tuning.

And Needle Bender, that second link above has a bunch of errors in it, you should find a better resource. Pretty much everything on that page is wrong.


The DB

 

[bob85]

NB we are not discussing how to feed the antenna or the pattern.

The topic is current distribution and phase.
You claim a 5/8 coaxial dipole needs some form of phase correction in order to have all radiating currents in phase unless you are missing something. Yes you are.

You claim any dipole longer than 1/4wave per leg has deconstructive currents.
That's incorrect its shorter than 2 x1/2wave

You claim a 2 x 1/2wave dipole has a cloverleaf pattern.
That's incorrect its shorter than 2 x 5/8wave

http://www.antenna-theory.com/antennas/dipole.php

 

[Needle Bender]

OK, so instead of current-fed, it's voltage-fed, providing two 1/2 wave current nodes.

One, positive, to the right,
one, negative, to the left.

How do they not cancel?

 

[bob85]

You got it NB its voltage fed like a 1/2wave endfed.

The article i posted says infinite impedance at the feedpoint but that's only true for an infinitely thin wire.
Real world conductors have a lower end impedance. Size matters.


How can the two halfs cancel when all currents are in phase ?
I don't understand your logic. They would only cancel if they were out of phase.

You don't get the theoretical 3dbd because the two 1/2waves don't have enough physical distance between them.
That's where a 2 x .64wave comes in.
It spaces the in phase 1/2waves to increase gain even though it has 1/8wave of your "deconstructive" currents in each leg.


Imagine its like a 1/2wave vs .64wave vertical but it has a real other half rather than an imaginary other half mirrored in the ground.

The advantage of the .64wave is it raises the usefull upper 1/2wave further above its imaginary other half.

Even though the lower 1/8wave is deconstructive the extra height of current maxima more than compensates up to .64wave where your deconstructive currents start to dominate and high angle lobes take over.
OR your cloverleaf takes over if its a dipole.

Of course that also means that just raising a 1/2wave to the same tip height or spacing the two 1/2wave dipole legs wider using a harnes will produce a bit more low angle gain because they have the extra height or spacing without the deconstructive currents.

Are we on the same page NB.
if not what part do you think i have all wrong ?

 

[Marconi]

OK, so instead of current-fed, it's voltage-fed, providing two 1/2 wave current nodes.

NB, I've heard guys talk about current nodes before and I'm always curious what part of the dipole they are pointing at. You or Bob can surely get this straight for me in my thinking. Is the current node(s) on the ends or in the middle?


I'm sure I've described this term wrong a few times in my posts.