Wall Loop 20m.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by PD7MAA, Feb 15, 2016.

ad: L-HROutlet
ad: l-rl
ad: L-rfparts
ad: Subscribe
ad: K3QNTad-1
ad: l-gcopper
  1. K3RW

    K3RW Ham Member QRZ Page

    Sorry for asking a dumb question--but how do you tune it for a given frequency? Use the rig SWR meter and tune the antenna knobs for a good match every time you move from, say 14.020 to 14.070? Or is the 300khz range good enough to tune it once with an antenna analyzer for a good match, and then leave it?

    I'm guessing for other bands follow the same concept but scale it down (?).

    Thanks for the great plans. I'm about to move back into an apartment for a few months while building a house, and this is a perfect solution for the interim and for future mountaintop stuff.
     
  2. PD7MAA

    PD7MAA Ham Member QRZ Page

    The loop is tuned with an analyser to the desired frequency in the 20m. band by changing the number of turns on the top section capacitor. My antenna is tuned for 14.200 MHz . for ssb use.
    However the bandwdth is more than 300KHz so tuning is not nessesary. If you use only cw then tune the antenna at 14.030 MHz .
    The capacitor at the feedpoint is 150pF in my situation but this depends of the wall or other nearby objects . Use a polivaricon to find the lowest SWR at your antenna and replcace it by a fixed Capacitor afterwards. I used 250V types capacitors .
    This antenna can be build on a fibreglas cross of 2.2 x 2.2 m. to create a field antenna in diamond shape.

    Have fun

    73 John
     
  3. PD7MAA

    PD7MAA Ham Member QRZ Page

    Loop circumference for other bands:

    20m. : 6m
    17m. : 4.8m
    15m. : 4.2m
    12m. : 3.5m
    10m. : 3m

    73 John
     
  4. DB9MAT

    DB9MAT Ham Member QRZ Page

    This is awesome. Will build one in the next couple of days :)

    Mind telling me with what length of wire to twist the matching section you started with?

    Edit: Well, reading certainly makes things easier. I guess it was 25cm ;)

    73 de Mathis DB9MAT
     
  5. PD7MAA

    PD7MAA Ham Member QRZ Page

    Hi Mathis

    I started with 20 cm. but it needed a tight twist. Better start with 25 cm .
    let us know your results.

    Have fun

    73 John
     
  6. DB9MAT

    DB9MAT Ham Member QRZ Page

  7. KE5IUN

    KE5IUN Ham Member QRZ Page

    If you find this antenna interesting, you check out Johns site at http://pa-11019.blogspot.ca
    lots of cool antennas and tuners
    73! Bill
     
  8. PD7MAA

    PD7MAA Ham Member QRZ Page

    The Z-Match could have a problem with a low impedance for it's designed for a high Z. A simple L - C tuner or a tuning capacitor at the feedpoint will do just fine.

    73 John
     
  9. K8JD

    K8JD Ham Member QRZ Page

    BIG antennas !
    I moved out to the far suburbs where I could put up B I G antennas, far from the house and power lines !!!!!
     
  10. SP3L

    SP3L Ham Member QRZ Page

    John,

    I have found this antenna recently and was really surprised how simple and clever it was. I tried to simulate it in the 4nec2 program. However, I simulated it in the open air, as I had no idea how to model a wall right behind it.

    I had to use 500 pF rather than 150 pF capacitor in order to get low enough SWR. I also got much smaller bandwidth than you have - see the graph below:

    upload_2016-3-21_16-28-26.png

    Evidently, the wall itself must play an important role in the antenna performance. I really do not like to be a "kill joy" but I am afraid that the big usable bandwidth you got means there are significant power losses in the wall.

    BTW, the top "capacitor" was 12.2 pF in my model. If 100 W was applied, there was 4,600 V present on this capacitance. So, be carefull everybody when trying out this antenna.

    Despite all that, I think this antenna is worth trying out. You can build it in no time and even if you warm up your wall a little bit, some RF power will still be radiated.

    73
    Jacek
     

Share This Page