Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Loop antenna math help  (Read 3913 times)
KC0RDG
Member

Posts: 126


WWW

Ignore
« on: December 07, 2005, 06:58:39 AM »

Hi all,

I am looking to make a vertically oriented horizontally polarized wire loop antenna.

In looking over the math, I have found the standard seems to be length = 1005/MHz, then divide by 4 to get your 4 sides.  However, in reading my ARRL Antenna Book, they have a formula "for determining the lengths for harmonic wires" (page 13-5).  This is in the Long-Wire chapter.  I am wondering if someone can help me with the math.  I am very terrible!  The formula is listed as:

Length(feet)= 984(N - 0.025) / f(MHz)  where N is the antenna length in wavelengths.

I am looking to design the antenna for 40m but would like it to be harmonic on as many bands as I can have it.  I will be using a tuner obviously but would like to get the cutting as close as I can.

Thank you!

Elijah
KC0RDG
Logged
WB2WIK
Member

Posts: 21836




Ignore
« Reply #1 on: December 07, 2005, 10:12:14 AM »

Start with 1005/F = length (ft) and trim from there.  There is no way in the world to have an absolute number for this, as it will depend upon exact height above the earth, the conductivity of the earth below it, and to some extent even surrounding structures (houses, trees, fences) in the antenna's near field.

This is why *everybody* making such a loop lets the corners "float" in the insulators, so overall loop length adjustments can easily be made.  If you want to make the loop really strong after it's been trimmed for resonance, you can lower it and then tie off the corners to their insulators using a second piece of wire at each location.

WB2WIK/6
Logged
N3BIF
Member

Posts: 1190




Ignore
« Reply #2 on: December 07, 2005, 01:10:35 PM »

 anything close to 135 feet, feed it with balanced line  and operate.
Logged
WB6BYU
Member

Posts: 18317




Ignore
« Reply #3 on: December 07, 2005, 01:41:42 PM »

All of the formulas are only estimates - and, at times,
not very accurate ones at that.  I've had loops come up
over 10% low in frequency if they are installed on the
roof of a house.  Try this link for a good discussion of
the subject:

http://www.cebik.com/wire/cut.html


Note that a wire will NOT have the same resonant frequency
in a loop that it does as a straight wire, also that the
wire diameter, insulation type, and height above ground
all have an effect.  I'd put up something close to 140'
and call it good - with the tuner you will be close enough.
Logged
KC0RDG
Member

Posts: 126


WWW

Ignore
« Reply #4 on: December 07, 2005, 02:32:17 PM »

Great, thanks for the info guys!

I have another question in reguards to feeding this antenna.

Here is my setup.

I have my shack in the basement.  My radio is midway in my basement and the feedline will run about 16 feet from my radio out a window and to the actual feedpoint.  The antenna feedpoint will be on the ground.  I will basically be mounting it next to my house (wood/stucco).  I have a tuner that I will be using.

I found this http://www.bloomington.in.us/~wh2t/loop.html saying to just attach your coax right to it, "Connect one end of the wire to the coax center and Connect the coax shield to the other end of the wire."  The article says you could use 75ohm coax since the antenna will have a impedence of around 100ohm.  I have also read to use a 4:1 balun and feed it with ladder line.  I'm new to all of this and want to do it right from the get go.

Radio-coax->tuner->coax 16 feet->4:1 balun->wire loop

Radio-coax->tuner->coax->wire loop

What way would be best?  Maybe it's neither?  Again, it will be on the ground (maybe a couple feet off at most), there will be a total run of about 16 feet from the radio to the feedpoint.  Thanks for your help.

Elijah
Logged
KC0RDG
Member

Posts: 126


WWW

Ignore
« Reply #5 on: December 07, 2005, 02:35:38 PM »

I should add that I am trying to basically homebrew this antenna http://www.qsl.net/dk9sq/verticae.htm
Logged
WB2WIK
Member

Posts: 21836




Ignore
« Reply #6 on: December 07, 2005, 03:34:47 PM »

All bets are off if you install this antenna "at ground level."  

Even a vertical loop fed for horizontal polarization requires elevation above ground, and the two articles you referenced don't say anything about feeding at ground level.  The first one actually discussed a horizontally polarized loop arranged in the conventional sense, parallel to the ground (so the whole loop is horizontal).

The parenthetic mention of installing a loop in the vertical plane in the form of a diamond shape, and feeding it at the top or bottom corner of the diamond to yield horizontal polarization does not imply that the bottom corner can be at ground level: It cannot be.

If this is your intention, I'd do something entirely different.

WB2WIK/6
Logged
KC0RDG
Member

Posts: 126


WWW

Ignore
« Reply #7 on: December 07, 2005, 04:07:23 PM »

Goes to show you want I know.  I was going to do something like this picture.  I have a dormer and I was going to put up a pole and use the house as a brace and then attach the wire to the pole, the house and then at ground level.

http://tinypic.com/ic0a6o.jpg

Got any other good ideas for a low noise DX antenna that you can install on a city lot?  Power lines run on the other side of my house both to the house and the a light on the street.  I have 1 tree in my backyard, it's maybe 40 feet tall.  I have no backyard to put down radials.
Logged
KC0RDG
Member

Posts: 126


WWW

Ignore
« Reply #8 on: December 07, 2005, 04:10:16 PM »

One of the other ideas I came up with was a folded dipole laying on the roof somehow.  I was looking at this one http://www.bwantennas.com/ama/fdipole.ama.htm  I want to homebrew something however and not spend $100+ on something that could be made.  I would like something low noise.  I don't care much about directivity.  I am just a Tech right now but planning on upgrading.  I am more of a listener than a talker.  I am currently using a Buddipole off the deck mounted on a painters pole.
Logged
WB2WIK
Member

Posts: 21836




Ignore
« Reply #9 on: December 08, 2005, 07:46:00 AM »

Compared to the Buddipole on a painter's pole, the loop you show should work GREAT!  But, it's still not going to be nearly as effective as it would if it were elevated.

Problem is, with horizontal polarization and the feedpoint at ground level, Mother Earth is shorting out a lot of your antenna.  It won't be "balanced," which is one of the advantages of a loop, because one half of it is very close to earth and the other half isn't.

I've never found loops to be any "lower noise" than most other antennas; not sure where all that got started.

A terminated folded dipole will of course be "lower noise" than an unterminated dipole because it has built-in loss.  For every dB loss an antenna has, it usually receives a dB less noise.  But it also transmits a dB less signal, so this is a tradeoff and demonstrates one of the advantages of using separate antennas for RX and TX: For TX you want something efficient and matched well; for RX, neither of those is particularly important -- you want something that hears signals better than it hears noise, even if "everything" is degraded by 10 or 20 dB.  On HF, many great transmitting antennas aren't the best for receiving, and vice-versa.

BTW, I live on a small lot (backyard is about 70% pool, spa, patio, walkways, not much land left) and have a utility pole carrying power distribution located right exactly in one corner of my back yard, close by enough that I had to be careful with tower placement to make sure I didn't have antennas rotating directly over or under the power lines.  I worked that out, but it was pretty close.  Despite all that, I don't have any "noise" problems, at least not any more than anybody else has around here.  The lines are quiet, and most of my "RF noise" that I hear on the HF bands is from stuff generated in my own household, including the zillion switchmode power supplies running practically everything in the house.  If I shut off the MAINS breaker to my home, the bands are extremely quiet, indicating all my noise is under my control.

You just might find the same thing, if you try that test...

WB2WIK/6



Logged
WB6BYU
Member

Posts: 18317




Ignore
« Reply #10 on: December 08, 2005, 12:14:22 PM »

Let me point you back to W4RNL's website again (an excellent
antenna resource.)

www.cebik.com/radio.html

Find his article on Vertically-Polarized, Self-Contained
Wire Antennas and read it.  This will give you a lot of
information about loops and different ways they can be
installed.

Basically, if you want to work DX, the best loop will be
vertically polarized; that is, fed 1/4 wave down the wire
from the highest point.  But this isn't a very good
multi-band antenna, as the radiation angles are too high
on the higher bands.  Or, if you have enough height, a
horizontal loop can provide good results, but that doesn't
seem to fit your situation as you have described it.

Then wander further down W4RNL's page to the article on
Vertically Oriented Multi-band antennas:

http://www.cebik.com/fdim/fdim5.html

I think you will find some good ideas there.


Meanwhile, regarding how to feed the antenna:  If you
have a 4 : 1 balun handy, try it and see what the SWR
is on various bands.  If you don't have one, just connect
the coax directly, or use twinlead instead.  Either way
it will go to a tuner in the shack (though, on occasion,
you may find a combination that doesn't need the tuner
on some bands.)

A common problem that newcomers have is that they think
there is ONE RIGHT WAY to do things in ham radio.  There
is no such thing.  True, some ways may work better than
others, but most choices are also predicated on your
specific environment, resources, and other constraints.
Things that may not be optimum in an ideal situation
often are pressed into service because they are the
most practical solution in the real world.
Logged
N7DM
Member

Posts: 672




Ignore
« Reply #11 on: December 08, 2005, 04:15:59 PM »

"WHAT N3BIF SAID"..........   Loops are not exact things...and you only get around 1.5 db gain from one that is high enough [half wave or better in the center of the area]  

But they sure are QUIET, effective antennas...
Logged
KC0RDG
Member

Posts: 126


WWW

Ignore
« Reply #12 on: December 09, 2005, 01:57:47 PM »

ok well i opted out for that and am going to be putting up a 40-10m windom antenna running from my roof, across the yard and to the garage.  we'll see how it works.

i think your right, alot of the "noise" is from within my house and not necessarily from the power lines.  i was under the impression a loop would be good, but i am going to give a windom a try and see how it works.

thanks all
Logged
N7DM
Member

Posts: 672




Ignore
« Reply #13 on: December 09, 2005, 03:02:09 PM »

Probably it will serve you well, OM.  Remember that we are still 'descending' into the pit between two Sunspot Cycle peaks.. and will be for a couple more YEARS.  That means more activity on the night-time, 'low' bands, and less on the sunny-hour, 'high' bands.  WHICH means it will much more important to get whatever sky-wire you choose...H I G H off the ground! Height is more important than anything... for a horizontal antenna. Consider a 4 inch PVC 'pole' to push up the current portion of that new aerial...

GL ES 73
Logged
WB2WIK
Member

Posts: 21836




Ignore
« Reply #14 on: December 09, 2005, 03:31:20 PM »

The old saying...

"Put your fire in the wire, and then get it higher"

...really works.

"Height" matters.  Ask basketball players.

WB2WIK/6
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!