Having looked in a couple of web sites and spoken to an experienced 136
khz loop designer/builder in Belgium the conclusion seems to be that the
sort of loop size used in Europe/UK in the large loop category does not
perform as well as a modest vertical system on TX and are only used by
those that do not have sufficient space for a vertical plus radials,
experimenters or those that have very high ground losses and  anything
is worth trying to put out a LF signal.
On RX the loop performs well in the desired direction of interest
(bi-directional) and is useful to those that have a noisy environment
where you can null out the offending noise.
In my case I live out in the country, where there are no overhead power
lines or lines of any sort for miles, all services in this area are
underground, therefore my vertical both on RX and TX gives excellent
results, with numerous reports confirmed across the Atlantic on 136
khz,  so I am able to compare the two systems, loop v vertical.
My loop parallel resonated on 136 khz gives about the same signal/noise
as the vertical, no discernable difference on RX. I have not tried it on
TX but according to others that have both loops and verticals say that
the vertical is superior every time. They cannot all be wrong
With this information to hand it would be pointless with my loop 4960 sq
feet trying to achieve the same results as my 120 feet vertical
supporting 4 x inv L antennas.
Someone else reporting here on email has already said Marconi got it
right first time hi
To try and get 1W erp from a loop would be a problem on the components
side, a vertical is bad enough. Low power experiments like across in the
USA where 1W to the antenna is the limit is a totally different story.
Some might not realise that to get 1W erp from a loop requires K/watts
of RF across the loop feed point. To achieve long haul results on LF one
has to aim for the max permitted power and an efficient vertical
antenna.
I have worked several stations this past few years transmitting on loops
but it has always been a struggle to hear them even at my quiet QTH,
some subsequently switched to verticals and that made a tremendous
difference.
My loop exercise has been interesting and I now have a spare RX antenna
suitably resonated for both 136 and 73 khz. bearing in mind the
bi-directional limitations. It radiates E/W.
If I had a truck load of high grade components, megawatt rated
insulators, capacitors and toroids I would be tempted to try
Transmitting.
73 de Mal/G3KEV