VK Logger Ham Radio Forum Archive

Hi all...

I was reading on the weekend, I read in reference to antennas for example that an antenna was only marginal for EME..

But back n those days....the only link modes were CW and SSB......my understanding is that even most antenna today will not do well on SSB....and for CW -6 dB is required......most of what I have read, most people only use a JT mode...

Now does this mean that some of those antennas from the past may work just as good as anything around today.....

I am mostly talking about Phased arrays...where they are low profile ..and have good gain...


John

Hi John

Some stations, including VK3ATN, (SK) used rhombics which were either several wavelengths long or stacked for EME. Steve VK3ZAZ will have more details.

73

Wayne VK4WDM

VK2FAK wrote:I am mostly talking about Phased arrays...where they are low profile ..and have good gain...

G'day John - interesting question.

I think that back in the days where extensive phased arrays were more common the competing long yagis (for the same gain) were relatively difficult to optimise w.r.t gain and bandwidth (antenna software was not as readily available as today). The difficulty with large phased arrays (relatively easier w.r.t. gain and bandwidth) is ensuring phase coherence over the many elements of the array (as told by builders of large radio astronomy arrays) - something which is relatively easier with smaller numbers of long yagis.

But, now there are many good designs for long yagis, I think the balance has well and truly tipped in favour of the yagi for high gain arrays - at least for the lower VHF bands.

Not to say you cannot find examples which refute the above by googling...

Actually - a phased array has been on the drawing board here. The reason being is that if the "down the block off a 70m cliff moon-rise" design isn't viable for EME (with the prospect of state-side contacts), the only alternative is a flat-on-its-back phased array within +/- 15 degrees of straight-up (albeit with contacts limited to my hemisphere).

The Short Backfire Antenna

Reading about these, I NEVER see a square one, why is it so?

These antenna have amazing aperture efficiency, so why not make an array?

At 23cm, about 600mm wide makes an array of four, quite attractive. 1.2m x 1.2m

15db gain each means about 20db for the array and, it's relatively flat, not a dish.

Does this make sense?

Is 20mm welded steel wire mesh good enough for the backplane and sides?

Alan VK2ZIW

Short Backfire Antenna (SBF)

Here is a publication that explains it all:
http://www.collectionscanada.ca/obj/s4/ ... NQ56152.pdf

Start at page 136
And, they build a 2x2 array of them.
Yes, they use waveguide feed, but the crossed dipoles wotk, just the same.
We at BMARC have 50m of 1/4" 70ohm heliax, to make the feed with.
(70 ohm video cable. Correction, ridged, not helical)

80

Alan VK2ZIW

VK2ZIW wrote:Short Backfire Antenna (SBF)

Here is a publication that explains it all:
http://www.collectionscanada.ca/obj/s4/ ... NQ56152.pdf

Try this:
http://www.collectionscanada.ca/obj/s4/ ... Q56152.pdf

(the 'f' at the end was missing from the original link).

VK4WDM wrote:Hi John

Some stations, including VK3ATN, (SK) used rhombics which were either several wavelengths long or stacked for EME. Steve VK3ZAZ will have more details.

73

Wayne VK4WDM

Yes ATN used Rhombics for 2M eme as did Chris VK5MC

The first and only 6M eme on cw that I am aware of from VK was done using the first series 2.5WL low angle yagi 12db gain made by Mike K6MYC and 1KW of CW.

The longest distance and Cycle 22 VK3 records were made using that array on F2 to both sides of the Atlantic approaching 17000km.

Dropping back 3db to a 9 ELEMENT produced or reduced the dx distances capable some years later and very few qsos were made over 15000K let alone 16000k.

However subjective assessment of antenna using qsos even eme ones depends on too many variables to say what is and is not better.

All my 6M eme was done using a very low phase noise receiver and a Preamp made by Lou WB6NMT.

And I might add hours and hours of input often without results.

All we had was 28.885 chat no loggers no clusters no real time prompting and no data modes.


2 cents