COMPACT - OFF-CENTER-FED DIPOLE "Project"

This 'Modified Version' of the Off-Center-Fed-Dipole (OCFD) Antenna is for people who do not have enough space to erect a full size multi-band antenna, yet still want the Same Performance as the full size OCFD antenna.


"Same Performance" means:


  • relatively good efficiency, and
  • relatively good bandwidth.


THE PROBLEM:  Most of the Space-Saving "Gimmick" antennas currently on the market are indeed small, but not efficient and have reduced bandwidth.


THE CHALLENGE: Find a way to reduce the required space, while maintaining good efficiency and good bandwidth!


A few year ago, Tom Rauch (W8JI) proposed an idea for a "Compact OCFD" antenna.  I finally found time to try out Tom's idea and my first impression of the antenna is VERY POSITIVE !!!

The Antenna used in this project is an Aerial-51 Model 807-HD (OCFD).  I designed it for personal use, but in the meantime it is built and sold by Spiderbeam GmbH and Spiderbeam-US as well as several of Spiderbeam's Resellers in other countries.
INFO on the Aerial-51 "807-HD":  https://www.aerial-51.com/model-807-xx/

THE CONCEPT:  Take a  known good multi-band antenna and bend it twice into the shape of a "extended-'C'" or  "Lazy 'U'", maintaining the full size of the original antenna.

Keeping the overall length "Full-Size" helps keep the antenna broad-banded.


The drawing on the right was my original plan.  Of course it did not turn out quite like I had planed - the wires droop. 

INITIAL MEASUREMENTS


I made these first measurements using a RigExpert AA-230 ZOOM analyzer, connected directly to the feedpoint.


As can be seen, all of the classic HF bands had relatively low SWR inside of the band, as did was 17m.


12m can be matched using an antenna tuner; 30m SWR was >10:1, so even when matched, probably won't be very efficient.


Bottom Line: it seems to work "similar" to the original antenna when erected in a straight line.

When we attach any length of coax to the antenna and then measure again, the results change, depending on the type and length of coax.


I have two pieces of RG-58 coax, each an electrical quarter wave-length long on 80m - used individually or in series. 


Although the SWR within the bands is similar to that measured at the feedpoint,  in between the bands the SWR is lower due to the loss in the coax.  Longer coax has more loss, as the curves suggest.

A look at the fundamental band gives us a better understanding of how the coax affects the SWR.


The frequencies shown are the frequency of minimum SWR, NOT actual resonance.


Note that the curve measured with an electrical half-wavelength does not coincide with the SWR at the feedpoint.  This is due to the feedpoint not being 50 Ohm!


With a good match at the feedpoint, the curves align quite well "within" the bands; albeit not in between bands!

I do not personally model antennas; I build, measure and test antennas . . .

"On The Air."


In the past 14 years I have made over 3000 field measurement on OCFD antennas using antenna analyzers to check the SWR curve across the band(s), and an RF Ammeter to measure Common Mode Current on the feedline.


Several friends do modelling for me, while sipping coffee or a beer, when I'm out in the field in the  Rain, Snow, High Wind, or sweating in the Sun, measuring antennas.


The model of this 80m C-OCFD predicts an excellent (more or less) "Omni-Directional" NVIS radiator.

So far, so good, but once erected, the antenna looked like a disaster.  Both of my fiberglass poles were bent, the wires (especially the top wire) were drooping quite a lot and were lower than shown in my "plan" drawing!   The next day I spent a couple of hours adding and adjusting some guy ropes  to the two fibeglass Spiderpoles.



NOW: The Top Wire is about 11m high, drooping a little in the middle, and the Lower Wire is drooping, being just 2m high at its lowest point.

TWO BIG QUESTIONS:

  1. How do the MEASUREMENTS compare to the same antenna strung in a straight line as a dipole or INV-V?
  2. How does its PERFORMANCE compare to the normal OCFD strung in a straight line?
  • This will be answered on a separat page after a couple months of On-The-Air testing.

FIELD MEASUREMENTS COMPARING THE C-OCFD TO AN OCFD ANTENNA.

(NOTE: This is the same OCFD antenna, just erected in two different configurations).

The graph on the right shows measurements  "at the feedpoint".


Most bands look fairly good with the C-OCFD, though not always as good as with the OCFD.



Never-the-less, it seems the C-OCFD, though a space-saver antenna, is still a good  antenna.

Note: the frequency of the 30m dip is outside of the band!

This graph shows us exactly what changed with the change on the fundamental band in the C-OCFD configuration.


The freq. of minimum SWR has shifted up the band by about 100 kHz.

We can easily shift this back to the middle of the Region-One band by increasing capacitance in the RMU by about 220 pF.

The Gallery on the right shows the SWR curves for the rest of the HF bands.


There will be DOWNLOADS of the measurement files of all of the measurements in ".asp" and ".s1p" format on a separate page.

         (Link Below)

The downloads will have more information, but only show one graph per file.  

Patience Please!  Rome wasn't destroyed in a single day!