H X & K aerials.

[Synchrodyne]

As far as I know, when a balanced aerial (dipole) is coupled into an unbalanced feeder (coax), the main potential problems are deterioration of the aerial’s polar diagram and so decreased attenuation of off-axis unwanted signals, and increased local interference pickup, particularly when the interference had a strong vertical polarization element. I suppose that that would mean that a horizontal aerial would have less rejection of vertically-polarized co-channel interfering signals, too. I suspect though that the installation of a balun would not have been a likely prognosis for any of these symptoms. Reorientation of the aerial or replacement with a more directional and/or higher gain aerial were probably the usual pathways resorted to. In some cases of course the potential problems might not have been actual problems. But it seems to me that the direct balanced-to-unbalanced connection came to be treated as normal and by inference technically correct, rather than as a “corner-cutting” and technically not-quite-right exercise that nevertheless could be gotten away with in some, perhaps many cases.

Cheers,

[colly0410]

Was stuck in a traffic jam on the main road into Bulwell today & noticed one of those halo FM aerials screwed to a wall mounted vertically, I'm sure it's not supposed to be done like that..

[Refugee]

Bodgers often fit those halos like that on the closest outside wall to the set they are to be used with.
I think they do it because they can't be bothered to get a longer ladder.

[colly0410]

Just been reading BBC monograph number 34 on their R+D site: They say that on the survey van aerials they experimented with using a balun & connecting the co-ax straight to the dipoles, but the difference in signal level was negligible so didn't bother using baluns. Wonder if that info got out to the aerial trade & they thought, "if the BBC can get away without using baluns so can we!"

[Synchrodyne]

Thanks. That BBC monograph is an interesting read.

Whilst the use of a balun was found to be not advantageous in the UHF aerial case, apparently it was so in the Band I case. There the dipole was connected to a balanced feeder which in turn fed a balun at the vehicle roof, with coaxial cable thereafter.

I suspect that the effects of using or not using a balun are situational. The potential problems that arise from connecting a balanced receiving aerial direct to an unbalanced (coaxial) feeder include degradation of directivity (obviously more important for highly directional aerials), and unwanted signal and interference pickup on the coaxial outer. Signal strength might not be much affected either way, although a balun itself might cause some loss. And as far as I know, the magnitude of the deleterious effects depends somewhat upon how many signal wavelengths there are between the aerial itself and the RF ground point of the coaxial outer. The latter I imagine would tend to be a fuzzy number at UHF. Given this, it seems likely that there could be many situations, with fixed domestic aerials, where the potential problems were not actual problems, so that the use of a balun would not make a material difference. But there would be others in which it might be beneficial.

The aerial trade was probably avoiding the use of baluns long before that BBC Monograph was published. The attached Belling & Lee advertisement from Wireless World 1952 August is illustrative. Mainly it was advocating the use of (unscreened) balanced 75R twin feeder as being more economical than the 75R coaxial type, but in the detail, B&L admits that whichever was used, there was balanced-to-unbalanced transposition somewhere in the system, which for exceptional cases could be corrected by the use of a balun. Screened 75R twin might have been a better choice, although it was costlier than 75R coaxial and lossier than unscreened 75R twin. I don’t think that B&L offered 75R screened twin as a specific TV aerial item, but elsewhere it did note that the screened twin offered as part of its antistatic AM radio aerial systems was also suitable for use as a TV feeder. So one assumes that it was close to 75R.

Cheers,

Steve

[Synchrodyne]

There was a good article on balanced and unbalanced feeders and the use of baluns in Wireless World 1950 December, copy attached. It was written by W.T. Cocking, which I think puts it into the authoritative class.

Cheers,

Steve

[Synchrodyne]

And here is a 1963 November advertisement from Belling Lee in which it asserts that the use of a balun at UHF was not necessary.

I suspect that this was done to counter other makers who were using baluns, probably including J Beam. As I understand it, J Beam’s earlier entry into the domestic TV aerial business was predicated upon the use of a balun in order to maintain the designed directivity of vertically polarized Band I TV aerials.

Cheers,

Steve

[Synchrodyne]

I don’t think that B&L offered 75R screened twin as a specific TV aerial item, but elsewhere it did note that the screened twin offered as part of its antistatic AM radio aerial systems was also suitable for use as a TV feeder.

Here is the B&L reference that shows that its L1221 screened twin antistatic (“Eliminoise”) AM radio feeder was also suitable for TV applications.

On the other hand, at the same time, BICC was offering both screened and unscreened twin as well as coaxial for TV aerial feeders.

Interesting is that unlike most, (or perhaps all) of the other UK aerial makers, BICC used a coaxial feeder rather than screened twin for its own antistatic AM radio aerial. It included an aerial-end matching transformer which I assume also served as a balun.

So on the basis of the above evidence, 75R coax was not yet in universal use for TV reception in the UK in the early 1950s. The twin feeders would have had something of a renaissance in the early years of UK FM broadcasting. Some of the early valved FM and FM-AM receivers from the major setmakers were fitted with 75R balanced aerial inputs. Early UK FM tuners though were often 75R coax only, sometimes 75R coax and 300R balanced. I think, but I am not sure, that UK TV-FM receivers usually had 75R coaxial aerial inputs for both TV and FM, whether separate or combined.

Cheers,

Steve

[colly0410]

Here in the Nottingham area most band 1 (BBC1) aerials were H or X shaped, however a few were K shaped. I'm wondering in the K shaped ones if the straight part was a dipole & sloping bits a director or were the sloping bits a bent dipole & straight part a reflector? The sloping bits sloped towards Sutton Coldfield.... When I lived in Langley Mill in Derbyshire we had a dipole on the side of the house & got a good picture, mind it was at the top of a big hill.

[colly0410]

Been reading in an old Practical Television mag (via that American site) that an ITA bod said "vertical polarisation at band 3 was best because of less ground reflections!" Hmm, if this is correct how come just about everywhere else in the world used horizontal polarisation for their main stations & only used vertical polarisation for low powered stations? I always thought ITV used the same polarisation as the BBC in the area used so cheaper combined band 1/3 aerials could be used by Mr & Mrs Viewer...

[Synchrodyne]

My understanding is that it was the GPO who directed the ITA to use the same polarization in any given service area as was used by the BBC. So effectively the ITA did not have a lot of choice. Perhaps the alleged lower level of ground reflections with vertical polarization - was it applicable only to Band III? - was something of a post facto rationalization, or making a virtue out of necessity.

Cheers,

Steve

[Focusdiode]

They are of course some "No mans land" areas like Chippenham, Wilts. This was on the roof at the house we lived in between late 1977-1982. The X is for Ch5 Wenvoe with the horizontally polarised BIII aerial for ATV Membury ch12. Some vertical BIII aerials around were for Southern Chillerton Down ch11.

Brian

[Synchrodyne]

Those “no man’s land” areas were probably inevitable given the .way in which national coverage was approached. The irony was that despite the longer each of the lower frequencies, the BBC service, largely shoe-horned into Band I where there were probably fewer channels than really needed, required more transmitters than did the ITA in Band III, where there were (relatively) abundant channels. Given that polarization variation was one way of addressing potential co-channel interference, then full “synchronization” of BBC and ITA polarizations was going to be unattainable.

Still, had ITA been allowed a free choice and had it chosen horizontal polarization for its main transmitters, it would have opened up the possibility of using combined Band II (FM)/Band III receiving aerials, at least in areas where the ITA Band III transmitter was near-sited to the BBC Band II transmitter or otherwise on a similar alignment for a given receiving site. Also, would a single-boom aerial with vertical Band I elements and horizontal Band III elements have worked?

The GPO directive on common polarization for ITA was noted in this item from WW 1954 September:

A few months earlier the GPO had announced that all additional BBC stations were to be in Band I, with no use of any Band III channels, contrary to the original planning:

The necessary clearing of Band III for ITA use was recorded in this WW 1955 July item:

And returning to aerials, quite early on BREMA had recommended a single 75R aerial input for TV receivers covering both Bands I and III:

Cheers,

Steve

[colly0410]

Been reading more old Practical Television mags (on the ARH site) & it seems there were two versions of the X aerial. One was a sort of bodged up H: the front part facing the TX formed a forwards bent dipole & the back part a backwards bent reflector. & the second a backwards bent dipole & a forwards bent director. Which one worked better though? No idea. Suppose it was hard to tell which was which from the pavement, unless you could see where the down-lead was connected. I presume both versions would be a bit cheaper to make than an H..

[Synchrodyne]

Well, the X required less material than the H, as it lacked the crossbar. Also, it had only one, admittedly more complex, fastener – at the centre of the X - as compared with H, which required three. So the X was probably cheaper to make than the H, and perhaps by a non-trivial amount in what may well have been a market that was more price-driven than performance-driven.

Insofar as directors were typically mounted closer to the dipole than reflectors in order to achieve the desired directivity results, the (dipole + director) form of the X might have been the better choice, as perforce the parasitic element was on average, quite close to the dipole, and probably too close to act as a really effective reflector.

The K-form aerial may have used even less metal than the X. On the other hand, it required an insulated mounting as well as reasonably complex fastener. One advantage may have been that it kept the supporting pole from being between the elements.

Whether any of these forms was mechanically more durable than the others is not clear.

I don’t imagine that the X and K had any overwhelming performance advantages as compared to the H, and the differences amongst them one way or the other might not have been very big. So lower cost may well have the main raison d’être for the X and K aerials. And then their makers had to invent technical sales support stories that may well have been based upon whatever (small) positive differences they could find.

Cheers,

Steve

[nuvistor]

Has posted earlier, the X aerial was much quicker and easier to install, I think that helped to make it popular with riggers. It was probably cheaper to make, not sure if that was relflected in its price though.
They came from Antiference (Antex) half assembled, the mast was fitted, the rods just turned them into position and tighten the wing nuts. Attach coax and that was ready for the mounting bracket, 5 minutes at most.

Frank

[Niall]

The X format is likely to be for physical rather than performance reasons. There are a number of 50 ohm designs around for ham radio use, down to higher HF frequencies (the ends of the elements often bent round about half way along and brought together to save space).
They are not seen for anything above low VHF as "proper" yagis are as convenient and probably perform better.

[colly0410]

Noticed in the old 50's/60's practical TV mags blurb on DIY aerials for bands 1 & 3 that none of them (that I've read so far) use baluns, just that on vertical polarised aerials the centre of the co-ax should be connected to the upper half of the dipole & braid to the lower half, on horisontal aerials it doesn't matter. Some interesting reading in those old PT mags via the ARH site, I'm up to 1961 now...

[Refugee]

I reckon this array is a little bit past its sell-by date but what do you think about the use by date if a rigger can be found?

[nuvistor]

If the junction box has not rotted away, remove the X and it would make a good DAB aerial.
Frank

[colly0410]

If you made the X parts a bit shorter it'd be OK for FM, I think. Wonder why you don't see X or K shaped aerials for FM?

[nuvistor]

If you made the X parts a bit shorter it'd be OK for FM, I think. Wonder why you don't see X or K shaped aerials for FM?

Perhaps with the rods being shorter for FM the size was not as much a problem as an H aerial on Band 1.
Did you notice the band 3 dipole had bent ends so as not to interfere with the X aerial when set in certain angles to each other.
If you want to see some really fancy aerials have a look at the American radio history web site, usually something in the Radio Electronics mag http://www.americanradiohistory.com/Rad ... r_Page.htm

Frank

[Refugee]

I spotted the "tweaked ends" on it too. It looks like it was manufactured as a package deal for both bands.
For DAB the band III would be a bit too directional unless all the stations you like are on one transmitter.
The solution for a DAB aerial on a portable is a telescopic back scratcher. These are just a little bit short for FM but work well for DAB.

[nuvistor]

I spotted the "tweaked ends" on it too. It looks like it was manufactured as a package deal for both bands.
For DAB the band III would be a bit too directional unless all the stations you like are on one transmitter.
The solution for a DAB aerial on a portable is a telescopic back scratcher. These are just a little bit short for FM but work well for DAB.

It was manufactured as a package, I helped fit many of this style for channel 2 and channel 9 in 1962. If I remember correctly the X would pivot through 270 degrees to cover many situations.
Agree on the band 3 being very directional, but I was thinking of a fringe area not local. The problem with most DAB sets is no aerial socket, I saw one advertised were the telescopic was unscrewed to reveal an F socket, not many about though.
Frank

[Refugee]

That one looks more the part.
It looks like one that would sell well in places with the TV stations at opposite ends of town.