wire beam antenna?

[Marconi]

Best to run the coax out inline with the boom in this case. The coax is likely to be a problematic factor in this setup. You are right, a balun might be needed in this type of setup. I would wait until you see if this will work at all as you have it figured out, before buying a balun. Maybe use the coil thing for starters.

Have you thought about how you are going to compare the effectivness of this antenna?

You also asked the following:

here are the results of my antenna "modeling":

gain:10.22 dbi
f/b: 11.07 db

the pattern looks "acceptable".

here's the part i need help with:
Zin=49.71 +j 13.07
what does this mean and how does it relate to feeding the antenna?

i interpret the Zin as the impedance of the antenna at the feedpoint. if that is right, then this antenna should be close to a good match without using a beta match.

what is the "j"? is this reactance?

what does it mean for me?

The J part of your resistance is the reactance part and the plus in front probably suggest that the antenna is a bit long electrically. However, I don't think it is enough to really worry a lot about at this point with that number. It might also mean that your center frequency may be a bit lower than expected if the actual value of J = +13.07 ohms. Exactally what software did you use to determine the value of resistance noted above?

You may also find using software, that depending on how it uses the earth affects, weather the coax is attached or not, and the height, all these can really affect the values for gain and rejection quite a bit. Does the software ask you questions about the earth, the coax, and the height at all?

 

[W5LZ]

The balun may be a bit 'premature' till you know what the input of the antenna is. Otherwise, how do you know if you need a "1:1", "4:1", or whatever balun? In general, a balun should help. That coil of coax, the 'choke' ought to do the same thing except for the impedances transformation thing.
I think Marconi is probably right about running the feed line down the 'boom' to get away from the antenna. It's one of those 'try it and see' thingys, pretty easy to change if needed.
- 'Doc

 

[Beetle]

Your Zin is so close to 50 ohms resistive it would be a waste of time and effort to try to adjust that part of the equation. The small amount of inductive reactance (indicated by the "plus j") shouldn't be a problem either. You might calculate how much capacitance you'd need to cancel it and install a small variable capacitor of suitable value right at the feed point. Or shorten the elements just a teeny bit.

If it were a "negative j", the reactance would be capacitive, and you'd either want to insert a small inductor, or lengthen the elements a small bit.

This is the part of radio that's inexpensive, that you can get down to the nuts and bolts (and wires, etc) with: Antenna Experimentation. Enjoy it. You might not know the function of every component in a radio, but fortunately, antennas aren't all that complex and the theory behind them hasn't changed significantly since Marconi's day. The ORIGINAL Marconi.

 

[loosecannon]

well alright!

i very much appreciate you guys answering my questions.

i guess its better to be a bit electrically long in my situation, as it will be easier to trim 1/2" at a time, than to add on wire. (yikes!)

very cool info on the reactance being inductive.
now i just need to find a formula that will allow me to calculate the variable cap i might need.
would an air variable cap from my old kenrich eagleVIII work?
not sure on their values, but wouldnt the LOAD cap be somewhat suitable? who knows.

marconi, of course i cant find the program i used to calculate this antenna when i need it.LOL
the answer is no, it did not ask me questions about height above ground, the earth itself, or the coax.

it allowed me to input the length of the elements, the spacing, the number of elements, and the elements diameter. (which for me is .064")
if you have some good antenna software i would appreciate it if you could run my numbers through it.
soil here is desert. (grass in my yards)
"boom" would be about 20' off the ground.
coax will be RG8x, and i will be using a 50' run of it. about 10 feet will be coiled up for the "choke balun".
thanks for the interest in this project. (check your PM's)

sorry there is no actual construction going on yet, but money has been really tight here and wont be back on top for another week or so.
will keep you "posted".
loosecannon

 

[Beetle]

Take the formula for finding capacitive reactance:

Xc = 1 divided by (2 pi) (frequency in Hz) (capacitance in farads)

and rewrite it to solve for capacitance since you already know Xc.

C = 1 divided by (2 pi) (freq in Hz) (Capacitive reactance in ohms)

Pi = 3.14, so 2 pi = 6.28.

Watch the decimal placement. Mega = 10^6. Pico = 10^-12.

 

[loosecannon]

man, high school algebra was a long time ago!

here is what i get. maybe someone here can interpret it for me. LOL

4.448893642 -7 (im guessing this means to the -7th power?)

help!
loosecannon

 

[Beetle]

Excellent job! I ran the calcs and compared them with several online reactance calculators (Google - your best friend!) and came up with a value of 448 pf (4.48 x 10^-10). I think you dropped a decimal place somewhere along the way, but hey!

(And I bet my high school algebra and electronics classes were longer ago than yours! ). I just use the formulas more than you probably do.

Trying to use the scientific calculator built in to windows can be confusing. I'd use my old slide rule, if I could find it!

 

[loosecannon]

thanks so much beetle!

dad is a civil engineer, and collects old slide rules.
he has quite a collection at this point.

448pF eh?

its a place to start!
im thinking that i could either buy a big air variable cap with a range centering around 450pF, or i could just buy a bunch of capacitors around that value.
if i went with the individual caps; what type would be best?
im thinking that silver mica caps would be good. who knows.

i really cant wait to get started on this, but alas, i will have to wait a few more days to buy the wire.

thanks again,
loosecannon

 

[Beetle]

Silver mica would probably work. The feedpoint of most antennas is a high-current, low-voltage point. 448 pf is a bit large for an air variable - most older AM tabletop radios used a tuning capacitor that maxed out around 365 pf (or 365 μμf, as it used to be written).