[UK-CONTEST] Antenna for SSB field day

[dave at g4buo.com]

Hi Rob

The 'Gravesend Loop' has been used in SSB field day by a number of groups.
Chris G3SJJ mentioned it in an earlier posting, but said you short the
bottom on all bands except 80m. That's not right, it needs to be
open-circuit on 20m so that it acts as a bi-square, with some gain
broadside.

The nice feature is you only need one 50ft support. It's a diamond loop
fed at the top with open-wire feeder, and a switch or relay at the bottom,
we usually have ours about 4ft above ground. The two corners are pulled
out with lines. The plane of the loop should be north-south.

It's certainly not true to say the only antenna anyone uses in the
restricted section of field day is a doublet. One group used trap dipoles
with success for several years. Others have used horizontal loops but the
Gravesend loop is worth considering for SSB FD.

Cheers, Dave G4BUO

> I'm sure it has been done but what about a vertical loop? Could be done
> with
> only two supports. Again, merely curious given that I've been charged with
> the antenna for this year's FD!
>
> 73, Rob
>
> -----Original Message-----
> From: uk-contest-bounces at contesting.com
> [mailto:uk-contest-bounces at contesting.com] On Behalf Of Chris G3SJJ
> Sent: 15 August 2010 21:51
> Cc: uk-contest at contesting.com
> Subject: Re: [UK-CONTEST] Antenna for SSB field day
>
>   Not wishing to keep this thread going longer, but reference a (sloping)
> V
> Beam you only need one mast, I am sure the Brizzzle guys could give us
> more info!
>
> The Rhombic was just a thought whilst mulling all of this over. Needs
> modelling but my thinking was that a bi-di rhombic, presumably without the
> termination R would be OK on 20/15/10, but would be high angle on 80/40.
> The
> horizontal loop on 80m seems to be popular these days as a a cloud warmer.
>
> Two masts about 100ft apart, feed point on one, termination R (or not) on
> the other. The other two corners pulled out on poly ropes.
>
> Dunno, but an interesting experiment for plotting on EZNEC and then in a
> field but we would need some sunspots to see if it actually works on
> 15/10m!
>
> 73 Chris G3SJJ
>
>
>
> On 15/08/2010 13:19, Ian White GM3SEK wrote:
>> Chris G3SJJ wrote:
>>> Ian, maybe very originally doublets figured strongly. One of the
>>> trophies for Low Power Contest has an old style Bell tent and two masts
>>> with a doublet stretched across, with sagging open wire feeder!
>>>
>> Yes, I've seen that one - it is a work of art, but hardly a model of
>> good antenna engineering!
>>
>>
>>> I think there is a greater awareness now of propagation spread, as you
>>> say, but I am not sure the basic rules need tinkering with to
>>> accommodate that. GM3POI won NFD Restricted section many times with
>>> well researched vertical system.
>>>
>> Having done NFD with the Orkney team for the past two years, it was
>> Clive's exploits with Dennis F5BVY that set me off on this trail. Our
>> club here at home has a need for a multiband vertical, and it would
>> definitely be a bonus if it could be used for NFDs in the Restricted
>> sections as well. But as soon as I started sketching out some
>> preliminary designs, I came up against these strange quirks in the
>> rules.
>>
>> Without giving away too many technical details, a vertical antenna would
>> obviously have to use the full allowable height, to maximise the
>> radiation resistance and feedpoint efficiency on the lower bands.
>>
>> But the current RSGB NFD rules require two *different* maximum heights
>> in the Restricted sections. For SSB NFD it's 15m, but why is it only 11m
>> for CW NFD - with Top Band to cover as well? That extra 4m makes a
>> notable difference for both horizontal and vertical antennas.
>>
>> Anyone who is using a doublet or similar can easily change the height
>> between 11m and 15m; but for vertical users this is a show-stopper.
>> Making full use of the maximum available height for each contest would
>> require a complete redesign, and the construction of two totally
>> different antennas.
>>
>> That discrepancy between the two RSGB NFDs needs to be looked at
>> urgently. (Hint: the height limit for the Eu/DARC CW NFD is 15m...)
>>
>>
>> Then we come to the rule about "two supports". Anyone who has chosen a
>> vertical, or even an inverted-V, is now limited to only *one* additional
>> support of any height whatsoever.  In many cases, by far the best
>> technical option would be to use the available mast sections to make two
>> shorter poles - but no, that is not allowed.
>>
>> I am not saying that anyone intended the rules to end up this way. What
>> we're seeing are the unintended consequences of a rule-making process
>> that said: "We know what the best antenna is for everybody - the doublet
>> - now what should the rules be?"
>>
>> Those rules have permitted some kinds of innovation [1] but other good
>> options have been closed off without even noticing they exist.
>>
>>
>>> It seems to me the concept of one single element antenna is established
>>> and a good leveller
>> "Established" doesn't automatically make it the best kind of leveller,
>> or mean it cannot be improved. We have already seen how easily the
>> "established" rules can fall victim to the Law of Unintended
>> Consequences.
>>
>> A far better way to limit the overall scale and resources of a
>> Restricted antenna - but this time, without limiting the technology -
>> would be to specify "one active element per band", a maximum overall
>> height (which should surely be 15m for both NFDs) and a new rule to
>> limit the total *length* of above-ground supports (specifically *not*
>> the total number).
>>
>>
>>
>>
>> [1] Surely the V-beam and the Rhombic weren't in the Restricted section?
>>