Arran Cameron:

Back in a more youthful era of my life I used to notice X shaped aerials now and again.  .  .


Omnidirectional FM aerials were one of those 'wonderful' products from the 1960s and 70s (when FM radio still had an air of luxury about it) with the advantage of being able to receive broadcasts from all directions in exchange for a gain slightly above 0dBi. Well, if you want an aerial that approximates an isotropic then don't expect a gain any higher than an isotropic! They had a secondary advantage of being easy for consumers to install as almost no alignment was necessary.


More recently, a local housing association decided that it would not provide terrestrial TV aerial sockets in some of its apartments and instead it had a communal satellite dish that could receive both Freesat and Sky. It resulted in a mixed reception (pun intended) with some tenants being very pleased with being able to receive more TV channels than on terrestrial and not having to install their own satellite dish if they wanted to watch satellite channels, but other tenants were unhappy and uptight about having to buy a Freesat receiver if they just wanted to watch terrestrial channels.  .  .


 

Yes, one still sees old X and H aerials, and multi-element Band 3 aerials from the ITV 405-line era. To send someone up onto the roof to take them down costs the householder money, for which he gains no real benefit, so that is why some remain deteriorating. Not many householders have the ability or the inclination to this job themselves.


But I do notice new buildings going up and new aerials being installed on them so not all circular aerials are relics from the past. The installation in the picture I provided seems fairly shiny and recent.


In theory, the horizontal gain of a circular omni aerial cannot be as much  as a single dipole vertically polarised. A horizontal single dipole has the same sensitivity perpendicular to it as a vertical single dipole, but its sensitivity tails off sinusoidally to next to nothing as we move round to a direction longitudinal to it. The sensitivity of a circular dipole can be no better than the mean of this. Bear in mind that the horizontal single dipole has sensitivity also in upward and downward directions (so is more affected by interference from aircraft). This also applies to a circular aerial. The vertical dipole has relatively little sensitivity to the (useless) vertical  directions and maximises its sensitivity horizontally. Based on this theory, my calculations indicate that the circular omni aerial has a gain of -2 dB relative to a single dipole vertical. Yagi arrays can achieve positive gain of several dB.


Serving a community with a single satellite dish does seem a good idea; one can receive all the terrestrial channels, the Sky subscription channels, radio, alternative BBC and ITV regions - the lot. But yes, tenants would need the Sky box or at least a Freesat receiver. As I said earlier, the provision of some terrestrial services, particularly FM radio, seems somewhat perfunctory.


As for single domestic users, my neighbour has removed all his terrestrial aerials and uses a satellite dish for everything. I have Sky but am not ready to do this just yet. Sky is very good but pricey. Extending Sky services to other rooms hikes up the subscription even more, whereas for terrestrial all you need is a distribution amplifier and some coax wiring. Satellite dishes are vulnerable to snow. One winter mine became snowed up, cutting off reception. Since it is not far from the ground I was able to clear the snow with the aid of a stepladder. If this happened to a dish high on a roof it would be a choice of going onto a snowy roof - dangerous and not recommended - or waiting for nature to take its course. Terrestrial aerials are not affected by snow.


I have taken down my Band 2 FM sound aerial but I reckon terrestrial television will be with us for a while yet.

Arran Cameron:

Back in a more youthful era of my life I used to notice X shaped aerials now and again.  .  .


Omnidirectional FM aerials were one of those 'wonderful' products from the 1960s and 70s (when FM radio still had an air of luxury about it) with the advantage of being able to receive broadcasts from all directions in exchange for a gain slightly above 0dBi. Well, if you want an aerial that approximates an isotropic then don't expect a gain any higher than an isotropic! They had a secondary advantage of being easy for consumers to install as almost no alignment was necessary.


More recently, a local housing association decided that it would not provide terrestrial TV aerial sockets in some of its apartments and instead it had a communal satellite dish that could receive both Freesat and Sky. It resulted in a mixed reception (pun intended) with some tenants being very pleased with being able to receive more TV channels than on terrestrial and not having to install their own satellite dish if they wanted to watch satellite channels, but other tenants were unhappy and uptight about having to buy a Freesat receiver if they just wanted to watch terrestrial channels.  .  .


 

Yes, one still sees old X and H aerials, and multi-element Band 3 aerials from the ITV 405-line era. To send someone up onto the roof to take them down costs the householder money, for which he gains no real benefit, so that is why some remain deteriorating. Not many householders have the ability or the inclination to this job themselves.


But I do notice new buildings going up and new aerials being installed on them so not all circular aerials are relics from the past. The installation in the picture I provided seems fairly shiny and recent.


In theory, the horizontal gain of a circular omni aerial cannot be as much  as a single dipole vertically polarised. A horizontal single dipole has the same sensitivity perpendicular to it as a vertical single dipole, but its sensitivity tails off sinusoidally to next to nothing as we move round to a direction longitudinal to it. The sensitivity of a circular dipole can be no better than the mean of this. Bear in mind that the horizontal single dipole has sensitivity also in upward and downward directions (so is more affected by interference from aircraft). This also applies to a circular aerial. The vertical dipole has relatively little sensitivity to the (useless) vertical  directions and maximises its sensitivity horizontally. Based on this theory, my calculations indicate that the circular omni aerial has a gain of -2 dB relative to a single dipole vertical. Yagi arrays can achieve positive gain of several dB.


Serving a community with a single satellite dish does seem a good idea; one can receive all the terrestrial channels, the Sky subscription channels, radio, alternative BBC and ITV regions - the lot. But yes, tenants would need the Sky box or at least a Freesat receiver. As I said earlier, the provision of some terrestrial services, particularly FM radio, seems somewhat perfunctory.


As for single domestic users, my neighbour has removed all his terrestrial aerials and uses a satellite dish for everything. I have Sky but am not ready to do this just yet. Sky is very good but pricey. Extending Sky services to other rooms hikes up the subscription even more, whereas for terrestrial all you need is a distribution amplifier and some coax wiring. Satellite dishes are vulnerable to snow. One winter mine became snowed up, cutting off reception. Since it is not far from the ground I was able to clear the snow with the aid of a stepladder. If this happened to a dish high on a roof it would be a choice of going onto a snowy roof - dangerous and not recommended - or waiting for nature to take its course. Terrestrial aerials are not affected by snow.


I have taken down my Band 2 FM sound aerial but I reckon terrestrial television will be with us for a while yet.