Some interesting discoveries

[kl7aj]

Well, I've been drilling down through all the cool features of 4nec2, and still learning things. For those of you looking for a good antenna modeling program, 4nec2 is the Duesenberg of software for this purpose....and it's FREE! (Thanks, Arie Voors).

One thing I like about 4NEC2 is that it actually includes Arctic Earth in its standard library of ground characteristics. In other words you can model antennas in MY universe....and the results very closely match what I actually observe up here.

Probably the most important thing I've learned is that, on 160 meters, with my ground conditions, the worst horizontal dipole will function better than the best vertical. I modeled my inverted L...four full-sized elevated resonant ground radials, a 60 foot vertical section, and about 80 feet of horizontal wire for the rest of the "L". The calculated efficiency is an abysmal 16%.

A full sized horizontal dipole at a mere 35 feet elevation comes out to 38% efficiency. An 80 meter dipole at the same height (excluding transmission line losses) STILL comes out to 31% efficiency on 160 meters!

All my efforts of pruning and tuning and radial stretching are pretty pointless. My 80 meter inverted vee works better. Not only in NEC modeling, but in practice.

So....the point is....well, there was a point here....somewhere.

Now you know.

eric

[kl7aj]

But my 160 vertical still has great EARS

[N4PRT]

The point is that Alaska is not the best place in the world for ham radio...:eek:

[AG3Y]

I'm just curious as all get-out, Eric. How about modeling a typical broadcast-type vertical, and see if the ground underneath it makes any difference ? Pick whatever band you want, but 80 or 160 would make the most sense, obviously !

1/4 wave vertical, with 120, quarter wave plus about 5% additional length, radials spread out evenly under the active element. Put a ground screen that is about .05 wavelength radius directly under the vertical active element.

The FCC has contended forever that that setup will perform almost exactly the same, regardless of the ground it is mounted upon. Can you prove them right, or wrong ?

"Inquiring minds want to know ! ! ! "

73, Jim

[kl7aj]

I'm just curious as all get-out, Eric. How about modeling a typical broadcast-type vertical, and see if the ground underneath it makes any difference ? Pick whatever band you want, but 80 or 160 would make the most sense, obviously !

1/4 wave vertical, with 120, quarter wave plus about 5% additional length, radials spread out evenly under the active element. Put a ground screen that is about .05 wavelength radius directly under the vertical active element.

The FCC has contended forever that that setup will perform almost exactly the same, regardless of the ground it is mounted upon. Can you prove them right, or wrong ?

"Inquiring minds want to know ! ! ! "


73, Jim


Hi Jim:

I've actually done this numerous times. I was director of engineering for KJNP, which had a two tower (each 1/2 wave at 1170khz...that's 420 feet!), so I had to keep tabs on this.

The efficiency in the NEAR FIELD was close to 97%, which any properly designed FCC approved antenna will be. The real issue is what happens 10-20 wavelengths away...or more. You have NO control over this, and yet this is what really determines your effectiveness, at leas for ground wave. One reason we went to half-wave towers is that they are much less susceptible to seasonal changes. However we did have to contend with HUGE voltages at the base....as you can imagine with 50KW! We sometimes had huge purple corona balls at the base...till we went to extra tall bas insulators. :)

eric

[AC0FP]

Eric,

Are you indicating the 4NEC2 program is better overall than EZNEC, or just for your Alaska location?

73,

Frank

[KA4DPO]

Hi Jim:

I've actually done this numerous times. I was director of engineering for KJNP, which had a two tower (each 1/2 wave at 1170khz...that's 420 feet!), so I had to keep tabs on this.

The efficiency in the NEAR FIELD was close to 97%, which any properly designed FCC approved antenna will be. The real issue is what happens 10-20 wavelengths away...or more. You have NO control over this, and yet this is what really determines your effectiveness, at leas for ground wave. One reason we went to half-wave towers is that they are much less susceptible to seasonal changes. However we did have to contend with HUGE voltages at the base....as you can imagine with 50KW! We sometimes had huge purple corona balls at the base...till we went to extra tall bas insulators. :)

eric

I've heard that in Alaska if you don't pre-heat the RF it just kind of oozes off the antenna. Is that true?:D

[W1RFI]

Poor ground also impacts the low-angle DX performance of the antenna, due to the ground that could be a hundred wavelengths away from the antenna. Over poor ground, a vertical is an antenna that radiates equally poorly in all directions. One can control the ohmic losses, but at low angles, over poor ground, much of the radiated energy will still be lost.

Ed, W1RFI

[KA4DPO]

In the case Eric describes where the station utilizes half wavelength verticals would the far field ground effect be the same as it would for quarter wave antennas?

Obviously for a quarter wave vertical antenna a full counterpoise is absolutely necessary for maximum current loading on the antenna but how necessary is it for a half wave vertical? Doesn't a half wave vertical also have a lower radiation angle than a quarter wave by virtue of the fact that ground current is inconsequential to current loading in the antenna?

The statement about half wave antennas being less prone to seasonal changes and changes in ground conductivity makes perfect sense.

[ab9lz]

I've heard that in Alaska if you don't pre-heat the RF it just kind of oozes off the antenna. Is that true?:D

Dunno about that, but I did light my vw beetle on fire (on a cold "spring" morning in Fairbanks) by trying to preheat the engine with a svea camp stove :(

73 m/4

[kl7aj]

In the case Eric describes where the station utilizes half wavelength verticals would the far field ground effect be the same as it would for quarter wave antennas?

but how necessary is it for a half wave vertical?

It's necessary because the FCC requires it. :)

[KE7HQY]

Eric,

Are you indicating the 4NEC2 program is better overall than EZNEC, or just for your Alaska location?

73,

Frank

Frank -

I can vouch for 4NEC2. Though I have not paid for the EZNEC registerered edition to compare it to 4nec2, 4nec2 is a **Full Featured** antenna modeling program. If it doesn't knock the socks of EZNEC, it certainly goes neck-n-neck with it. And you can't beat the price either, free! This is one of those things where you have great software at the best price (free!), and it's constantly being updated.

[N4AUD]

Thanks for telling us about this software. I wasn't aware of it, but I've downloaded and installed it. I haven't had a chance to use it yet though.

[AC0FP]

The point is that Alaska is not the best place in the world for ham radio...:eek:

What I get is Alaska is a great place for amateur radio, if you have a beam that can only be mounted at 20' it will still kick butt for DX.:)

[AG3Y]

But then, there's those darn auroras ! ! !

;) :p