Floating Antenna Ideas..

[n3lrx]

I've been thinking of different ways to load the 160/80 dipole I'm building.

To start with it's a 160m 1/2 wave folded over. There is an air gap at the ends.

What I'm wondering about is a few feeding techniques. One I ideal I had is like this..

Idea #1

full length 75m 1/2 wave no breaks.
___________________________________
_______________  __________________ - Open ends radiator separation 12" apart.
  leg 1        | |  leg 2
               | |
               | |- Open wire Feedline

I suspect it will have some directional properties but will it be an advantage or disadvantage being as the driven and parasitic are so close? (Antenna will also be a horizontal L)


#2 Idea is:
                    Leg 1 fed opposite leg 2
 _______________   __________________
|_______________ / __________________| - shorted ends 12" apart.
               | |
 Leg 2 fed     | |- Open Wire Feedline
opposite leg 1

Kind of criss cross feeding so it's like an S or Z lying down flat.


Yet another, Idea #3:

               
75M Trap so it's a full wave loop at 75 and an open ended dipole on 160.
                 |                   
                 v

 _______________UuU_________________
|_______________  __________________|- Shorted ends 12" apart.
                | |
                | |- Open Wire Feedline


Yet one more.. Idea 4


                | | - Shorting stub made of ladder line. Goes down to a relay box
                | |   on the roof, shorted to make loop, opened to make folded                                                                   
                | |   dipole on 160
 _______________| |_________________
|_______________  __________________| - Shorted ends 12" apart.
               | |
               | |- Open wire Feedline
               | |
               | |


I have no idea how either of these ideas will work. Has anyone ever tried it? If so are there any advantages and/or disadvantages?

[N8LGU]

    I don't have any computer analysis programs but it would be my guess that these would work on 80M at about 150 ohms but on 160M the "foldback" would cause nearly 100% cancellation. Also, the transmittion line would get involved on 160M (might be a good thing?). Just a guess...

[n3lrx]

I was wondering about that myself.

How would the cross feed idea work? Would it add some directional properties to it? Or would it do nothing?

I'm hoping for the later of the two. My main reason for cross feeding it is to flatten out the antenna so the feedline weight is dispersed equally instead of all being on one side.

As for software, I found some Linux software (Open Source) that does antenna analysis but I can't remember where it was.. Would be nice if I could find it again.

I've also got the Antenna Book on my gotta have list for the up coming hamfest..
 

[Tom WA3KLR]

Randy,

It appears that you have enough property for a 75 meter half-wave wire.  I favor getting a center-fed antenna as high and as long as possible and have vertical end wires that come down close to the ground, as safety permits, making your total wire as close to a half-wave at 160 that you can.  This will get the feed point impedance as high as possible for 160 meter operation, which helps keep transmission line and tuner losses lower.

  _________________   _________________
  |                            | |                              |
  |                            | |- Home-brew            | 
  |                            | |  Open wire              |
  |                            | |  feedline                  |
                               | |


If you want a good but not too expensive antenna program, I recommend the EZNEC 4.0 at $89.

There were 2 topics here recently on the AM Forum that dealt with the electrically short 160 meter antennas.  It would be good to read through them if you already haven’t:

160 Meter Transmission Line Study - SWR and Losses 
http://amfone.net/Amforum/index.php?topic=9122.0

A 60' flat top for 160 and 75m!
http://amfone.net/Amforum/index.php?topic=9181.0