Feb 29, 2012

Beverage On Ground (BOG)

Lowbanders cannot operate without the availability of a dedicated receiving antenna. One that does suppress local noise, suppress sky noise from certain directions and suppress QRM from certain directions. In fact, all it does is deleting signals and/or band noise you do not want. What remains is the dx, coming from that single direction that was left unattended. That is really it. 

The ability, to delete all directions but one, is put in two figures of merit; MDF and RDF(refer ON4UN’s book). In short, a kind of horsepower if you will, to distinguish the good from the better receiving antennas. And just because humanoids simply must quantify everything

I refuse to go in detail about RDF and MDF, because they represent computer models from antenna modeling programs, which by principle, cannot handle close to ground antennas, or more specifically, cannot handle real ground at all. Until someone takes a helicopter and runs zillions of circles at different heights around a lowband receiving antenna, measuring its pattern, I rather use my ears instead. 

And then again; what elevation angle is required at which moment? Too many parameters. Better have a few antennas more at your hand!

Setting up station @PI4TUE

The lowband receiving antenna is a whole lot different from multi-element VHF yagis, where forward gain is simply the most important figure of merit.
But, there is also a common factor between them; the “better” the antenna, the smaller the forward lobe in degrees azimuth/elevation. Where “better” has a total different interpretation; the VHF yagi needs the small lobe to collect all tiny weak signal particles from the dx and to a lesser extent, avoid signals from the side and rear. Lowband receiving antennas mainly concentrate on deleting signals from all unwanted directions and to a lesser extent collect the weak signal from that direction which remains.

Antenna computer models?

I shit on virtual, calculated dB’s.

In the end it all boils down to achieve a S/N ratio where you can make the QSO yes or no.

There’s no free lunch.
A decent lowband receiving antenna (like multi phase staggered beverages) requires real estate and/or extensive hardware and trimming (6-9 Circle). A physical shortcut like Flag/Pennants/K9AY/EWE, or even the BOG is a choice mainly based on the will to put up any RX antenna to receive at least something. Some work better than others, depending on the environment. And then there is man made noise for which vertical polarized antennas seem more sensitive. Or is it not?

You need to experiment and find what works best for you.

On a site note; even a short vertical, suppresses high angle signals thanks to its theoretical doughnut shaped pattern And being less susceptible to local noise because of its short length, this IS a valid dx receiving antenna. The total opposite is a short backyard NVIS covering high angles only. Here you have it; two RX antennas which fit even the smallest gardens!

Need proof? Find my result in 2012 CQ160 CW Low Power

ARRL DX CW contest 2012 @ PI4TUE club station
During last week’s ARRL DX CW contest weekend at our PI4TUE club-station, we (again) were in desperate need of a working RX antenna and had little time to install it in the usual public space.
The PI4TUE station is located in a building at the Eindhoven University of Technology.
It’s nice to have antennas at 70m above street level, but the local noise level is very high. Let me emphasize that again; for the lowbands extremely high!
Both on the roof and on ground level (up to a distance of 150m from the buildings), we have experimented for some 10 years with the following receiving antennas: K9AY, KAZ Delta RX(only on roof), magnetic loop (s), short tuned verticals ... etc. All including extensive common mode filtering. 

Nothing ever offered any S/N advantage compared to the available TX antennas; full size 40m dipole and vertical @65m, full wave 80m loop @73m and two full size 160m 0.25 slopers @63m.
Because of limited available real estate, beverages have not been tried thus far. Also, there are some large rusted fences along the property perimeters.

But Topbanders never give up. So this year it was time to try a BOG.

Do or do not, there is no try!
On a piece of lawn, facing the USA, 8 plastic fence posts were put in the grass. A 70mtr (max available space) long 0.5mm dia insulated stranded copper wire was linked on the bottom of the posts. At both sides a copper pipe was hammered 1 m deep into the ground, hoping NOT to meet any local wiring. KFC! From our shack at the 13th floor, 150m 75Ohm SAT coax + another 50m RG58, across several roofs, cross-overs between buildings (which required informing security, asking them NOT to shoot us from the roof) was used to connect to our BOG through a 1:4 transformer on a BN73-202 binocular. There were no 50/75 transformers installed. This was never tried before, not permanent and we had about 3 hours available.

On average, the antenna wire was at a height of 3 to 12 cm across the lawn.

Why not directly on the ground? Well, according to theory, directly on the ground should have an electrical "lengthening" effect (which is wanted). But on higher bands (40m) such can introduce a direction reversing effect (not wanted). Also, flat on the ground the signal is supposed to drop several dozen of dB’s. Well, theory is one thing, and often ensures that we hopelessly remain staring at computer models. 

Instead of debating, better start building the darn thing!

Since we really needed *any working* receiving antenna on 160/80/40 and there was no time for experimenting anyhow, the wire was deliberately hung 2 inches directly above the ground.
Termination at start was 390 ohms, and later changed to 195 until the analyzer measured a flat SWR curve up to 10 MHz.

At the TRX end a HD version common mode filter was connected: 17 turns of RG174 on a #77 ring core, followed by a galvanic separation using 2x2 turns on two BN73-202 binoculars, with the secondary separately routed in Teflon tubes. Over the top? Yes, maybe, but since all our previous effort only offered noise and I use this kind of filter successfully at home. 

On Friday afternoon, around 17UTC a quick check at BCB frequencies well known to us (1584, 4015, 6995 etc) all clearly showed F/B and F/S performance, which was promising!

"You sure you don't hit anything ??"

Blue wire near ground IS our BOG

It works!
Those of you who participated, know this was a weekend with some of the worst lowband condx for years. Even more reason to have any decent RX antenna available.
In the contest, our antenna was a success!

None of the 20 worked USA mults on 160m, delivered by the BOG, could be heard on the full size TX sloper antenna. On 80m this was the best receiving antenna ever. From a quiet S1 background noise, stations appeared out of nowhere with S1 - S5. Again, most of them were no QSO or just not there on the TX full wave 80m horizontal loop @73mtr AGL.  On 40m about 30% was better than the quarter wave TX vertical at 65m height. In most cases the S/N was better not only because of the lack of constant cracking noise from the TX vertical.

Why did we succeed this time? Because the BOG is only active in the horizontal plane, the local man made noise is perhaps largely ignored? That is supposed to be a myth. So I rather stick to the remaining fact that on a specific lawn on which several other types of RX antennas have been tried, this one did work FB.

We do have experience with different kinds of 200-300m phase staggered Beverage antennas on (large) outdoor terrain for many years. Any kind of BOG is a poor performer in comparison.
But this thing sure has potential at (small) residential areas.

What is next?
No hallelujah yet. We will soon make some additional attempts out on the free field, before moving it along the property of our neighbors.

The internet offers very little data on the BOG.
Please, can any of you share their experience on balancing BOG parameters like height, termination, length, etc.?

73 Mark, PA5MW (PI4TUE)


PC5A said...

Thanks Mark for sharing the details.
Let's try to raise the BOG a little bit for ARRL SSB and see what happens...

Anonymous said...

Mark moet je maar een opsporen ..heb erg genoten van dit stukje tekst..ben zelf naarstig aan het zoeken hoe het beste uit de 160m te halen, en dan met name de ontvangst...gebruik een verticaal, met goede eigenschappen, maar helaas krijg niet genoeg ontvangst binnen om nou net even die dx te maken.(ewe,en magnetic loop 2m rotateble)helaas is mijn tuin 35x12m.maar erg leuk humoristisch stuk, met de beleving zoals ik het ook zie...73e pa3bha arno