The current solar minimum prompted me to explore the feasability to design and construct a "low VHF" interferometer, which could
produce some useful results for at least a couple of years until the sun "fires-up" again. During active solar conditions ionospheric trans-
parency will be poor and lots of distant TV carrier signals are likely to ruin records for hours at a time.
However, the antenna could then be scaled down to a single system to record solar noise bursts during flare activity.
The BIG problem with (relatively)low frequency operation is the poor spatial resolution offered by physically feasable antenna systems.
Dishes are totally impractical, requiring > 50m dia., even Yagi arrays would have to be impressively large, similarly to the ones used by hams for 50MHz EME. Look at K6MYC's 50 MHz EME antenna system, not for the faint-hearted, especially when considering we have
to construct 2 such systems to make an interferometer !
The flux densities of cosmic radio sources are quite high at low VHF frequencies, so I thought it may be possible to use "wire antennas",
rather than huge Yagis. Weaker sources may be too difficult to detect anyway, due to the high galactic background temperature at 48MHz.
Studying over a few antenna books, I decided to make a design based on full wave dipoles, as shown below :
The basic idea was to fabricate a couple of "wire beams" , consisting of 4 full wave dipoles ,which are end-fed in phase by 1/4 wave long
stubs. This results in a 1/2 wave phase delay, which is what is required . Additionally we have a set of 1/2 wave reflectors and directors..
The whole system looks then like 4x 3el. Yagis side-by-side. And I still think there was not much wrong with that theory.........
In practive however the antenna systems exhibited strange behaviour ,like being much more sensitive to other directions than what they were
pointed to. The next pic shows the general arrangement and the way the array could be "steered" :
To cut the long story short, this approach was not working properly due to the extreme ground proximity of the antenna elements.
It was then decided to utilize the problem, rather than fight it. Only the centre, "driven" elements are now being used, but mounted off-
center, which facilitates changing the elevation of the antenna wire. The antennas are "all-sky" , at least in declination. A reasonable
amount of discrimination in declination is achieved by raising and lowering the distance of the antenna wire from the ground. This gives
very useful gains for different declinations. For max. gain overhead (-40.....-10deg.declination) the wire is 1/8 wave from the ground (~80cm). Approx. 1/4 wave from the ground for -60 and 0 deg.declination, and 3/8 wave for -80 and +30deg..declinations
This was inferred from the antenna radiation diagrams as below : (ARRL Antenna Handbook)
The antenna "wire" is now configured as shown below, together with its (theoretical) azimuthal radiation pattern : (ARRL Antenna Handbook).
As it turned out, 48MHz flux densities are really enormous and also quite different from what I'm used to at 1400MHz. Thermal sources ,
which are often dominant on 1400MHz, are usually "invisible" at 48MHz. To this extent Cyg-A is much stronger than the Sun , at least
until the Sun gets active again. Another surprise is the difficulty of recording Sag_A , something not entirely clear to me, because it should
record very well, being a synchrotron radiation mechanism. The telescope is confusion limited, sources recorded have to be identified by
their exact fringe duration, which is 732 seconds (12.2mins) at 0 deg.declination, increasing to 1464 secs at 60 deg. declination.
Interferometer fringe width is 3.05deg, the antenna system spacing E - W is 117.8m (= 18.8 wavelengths)..
Fringe beat frequency phenomena are commonly observed and provide special interpretative challenges.
But in the end it is truly astounding that one can record with just two lengths of wire deep space radio sources, and quite comfortably too.
The signals are of such intensity that it should be possible to record in single transits several of the major sources with just a couple of 1/2 wave dipoles , or even Ground Plane antennas, instead of the collinear wire array.
