Antenna shapes baffle me.

[AlmostHandy]

I'm confused at the myriad of shapes antennas can take, and I am unable to find any really good (layman) explanation for why they are shaped the way they are.

I understand that antenna theory is quite complicated, so if I can't get a simple answer, I understand.

Ok, here it goes.

Can any antenna design be enlarged or reduced to change the frequency? (or) How are certain shapes or configurations limited in scalability?

Let's say I have a design for a 2.4/5 ghz directional antenna. Can I change the scale of the antenna to receive a different frequency, such as 800mhz, or 100 mhz? Is it simply a matter of element size?

How does changing the size of an antenna affect SWR and Impedence? Is there a way to compensate?

 

[davidmc36]

MMANA -GAL is a program that you can put values into and then change things and see what it would do. I just downloaded in the other day and have not learned how to use it yet. It may help you.

http://mmhamsoft.amateur-radio.ca/

 

[AlmostHandy]

Far out! I love new toys!

I'll check it out for sure!

 

[majoco]

Almost. Your humble discone that a lot of people use for scanning because it has a wide bandwidth can also be scaled up to be used on HF frequencies too, although the centre mast gets a bit high at 3MHz! Still has the typical 10:1 frequency ratio and SWR remains low.

 

[kb2vxa]

In that case maybe this will be of interest. http://www.titanmissilemuseum.org/

 

[AlmostHandy]

Ok, that's huge.

Is it effective? Is it more effective than any other type of HF tuned antenna?

 

[N_Jay]

Almost. Your humble discone that a lot of people use for scanning because it has a wide bandwidth can also be scaled up to be used on HF frequencies too, although the centre mast gets a bit high at 3MHz! Still has the typical 10:1 frequency ratio and SWR remains low.

10:1 is pushing it.

More typical is 3:1 to 4:1 spread, with a well built antenna reaching 5:1 (usually at lower frequencies)

 

[AlmostHandy]

Ok, couple of things.

1. What are those ratios?
2. Why is a discone shaped the way it is?

 

[k9rzz]

GOOGLE !!

http://www.radio-electronics.com/info/antennas/wideband/discone.php

 

[majoco]

Wiki has a good explanation of discones and says that it has a received frequency ratio of 'up to' 10:1 but a transmitter power ratio of 'up to' 5:1 for a well designed discone wirh an SWR of less than 2:1.

The 'ratio' we are talking about is the highest to lowest frequency. Most of the discones advertised for general scanner coverage are said to have a 10:1 ratio, so if the lowest frequency useable is 50MHz, then the highest will be 500MHz, but "your mileage may vary" and anyway how could you prove it? Some of the HF antennas that look like discones are actually half of a 'biconical', that is they are mounted vertically, so the other half of the biconical is actually the ground which needs a proper earth mat to keep its broadband characteristics. Tuned radials would obviously make it a tuned system and so lessen the bandwidth.

 

[N_Jay]

Ok, couple of things.

1. What are those ratios?
2. Why is a discone shaped the way it is?

Lowest frequency : highest frequency, ratio.

I would have to dig out a waves and fields book. (And I am too lazy):lol:

 

[DaveH]

10:1 is pushing it.

More typical is 3:1 to 4:1 spread, with a well built antenna reaching 5:1 (usually at lower frequencies)

My understanding is that while the discone can provide a fairly good impedence match
over a wide range, the usefulness is limited to about a 4:1 or 5:1 above the lowest
design frequency. It is because the radiation angle increases with frequency,
up to 45 degrees or so at 5:1, which is usually what users don't want/need.

This is a reason why for 400/850Mhz use, a discone cut for 400MHz is better than
one (say) cut for 100MHz lowest frequency.

Dave

 

[DaveH]

Can any antenna design be enlarged or reduced to change the frequency? (or) How are certain shapes or configurations limited in scalability?

Let's say I have a design for a 2.4/5 ghz directional antenna. Can I change the scale of the antenna to receive a different frequency, such as 800mhz, or 100 mhz? Is it simply a matter of element size?

How does changing the size of an antenna affect SWR and Impedence? Is there a way to compensate?

Antenna designs can be scaled for frequency, but not just antenna lengths but
spacing between elements, and the thickness of the elements also come into
play.

It may or may not be possible/practical to modify a particular antenna
after the fact for a different frequency band. Some antennas use a matching
network which is frequency-dependent, which also has to be taken into
account. I think of a 4:1 coax balun, which is a 1/2 wavelength at a certain
frequency (for example).

Dave

 

[breadtrk]

In that case maybe this will be of interest. http://www.titanmissilemuseum.org/

I had to deal with that in Ground Radio in the USAF. You want a math headache? Decide to build a 5ft tall concrete permanent base for that thing and then do the math to get the new correct length of the stay wires. They would only approve the modification if we gave them exact measurements. This was pre computer days, all we had was a TI Calculator, a bunch of young guys who had not been to college, and a MasterSgt that was being inflexible that would not let us just go build it and physically figure out the length.

The good old days.

 

[kb2vxa]

"Is it effective? Is it more effective than any other type of HF tuned antenna?"

Welllllllll... almost Almost. Just because it's cut for HF doesn't make it any different from those we use on our scanners. If you've been on this site long enough I shouldn't have to tell you how it stacks up. KG6WOU says; "In a nutshell, it was quite fun to use. They claim no gain, but it sure seemed way superior to my dipoles here at home. Once I mentioned where I was, I got a mini pile-up going and worked 24 stations in an hour of casual operation."

Bread, that's radio school for you regardless the branch of the military. I got some of my education from the Navy but not In the Navy. Too bad I lost my Nav Pers folders a while back, excellent reference material for the slide rule generation. Lucky you, a calculator, just plug in the numbers but I had to work the equations and a slide rule is like a baseball bat, it'll get you there but will the ball land near first base, second base or somewhere in the outfield? Maybe that's where the phrase "good enough for government work" came from. (;->)

Oh BTW, in the real world it still looks good on paper but when you build it it'll never work. That's why I'm a cut and prune man, cut a little long and trim it back a little at a time. That reminds me of an old joke about the cheapskate Indian and the circumcision. Every price quote was met with the same phrase; "Ug, too much." Finally he does it himself with his tomahawk... you can guess the punch line.

 

[breadtrk]

We talked around the world on one of those and an Orthogonal


i don't want to hijack the thread but will say that the big cone I had to play with worked very well

Slide rule? Whats that?.

 

[AlmostHandy]

Wow. This new hobby of mine is really quite overwhelming.

Google helps, and I've found some really cool sites with information, but a lot of it makes me feel pretty dumb.

This book on scribd has some really good explanations of antenna shapes.

 

[AlmostHandy]

That's why I'm a cut and prune man, cut a little long and trim it back a little at a time.

Ok, Concerning this, and considering this antenna design, how critical is the length of each element? IAlso, what is the deal with the little curly cue of wire at the end of each element and radial? Do you think that's necessary?


Oh, one more thing, do any of the three elements affect the others? As in, when I start trimming a little at a time, should I start on the main element, and work smaller, or start on the small one, and work up, or does it even matter?

Hey, I just realized you actually wrote the book on SO-239 ground planes. Is that one of your designs?

 

[breadtrk]

I think the curly Q might have some capacitive component, but the "Q" is really to keep you from poking your eye out.

 

[AlmostHandy]

ROFL!! Nice. Thank you for that. I think I'll keep them on, and keep both eyes.