153.35MHz and 153.025MHz? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Biggles · 27th Jan 2018, 3:07 pm

Some paging transmitter antennas were (are?) a long white pole usually about two or three metres in length and probably co-linear by design. Typically you would see more than one mounted on a tower, presumably one antenna for each channel in use. I think the transmitters were anything up to 100 watts output (as opposed to the usual 25 Watts from a PMR transmitter) and when fed into a co-linear this would provide quite a large signal. The average radio scanner receiver would typically suffer from breakthrough, if within a mile or two of the transmitter. I presume that this is due to a combination of sheer overload because of the transmit power, and the wide front ends typically used in scanners. I never worked on the national paging network but regularly saw them in shared site accommodation. We once had breakthrough on our main receivers from a co-sited paging network, but I think some notch filters sorted that if I remember correctly.
Alan.

G6Tanuki · 27th Jan 2018, 6:37 pm

In times-past some of my repeaters shared masts with VHF pagers: the typical pager-antenna was a folded-dipole. On a "square" lattice mast there would usually be one such antenna on each corner, usually some way down from the top of the mast [cheaper to rent than at the top!] - depending on local/semi-local terrain sometimes you'd get two folded-dipoles on one corner if they needed to radiate more in a particular direction.

Breakthrough was always a problem: plenty of the pager transmitters ran 500W and on a couple of occasions we needed to use 'barrel' cavity-filters [high-Q things that looked like a beer-barrel - inside the barrel there was a Silver-plated 1/4-wave element with an adjustable two-metal-plates capacitor to set the resonance] to keep the pager-RF out. This sort of thing: https://www.telewave.com/2017/07/31/...avity-filters/

On less-problematic sites we used multiple 1/4-wave and 1/2-wave coaxial 'stubs' made from LDF4-50A Heliax, which could easily give 70dB attenuation at the desired 'notch'.

dodgy-dxer · 27th Jan 2018, 8:29 pm

Quote:

Breakthrough was always a problem: plenty of the pager transmitters ran 500W and on a couple of occasions we needed to use 'barrel' cavity-filters [high-Q things that looked like a beer-barrel - inside the barrel there was a Silver-plated 1/4-wave element with an adjustable two-metal-plates capacitor to set the resonance] to keep the pager-RF out. This sort of thing: https://www.telewave.com/2017/07/31/...avity-filters/

Telewave didn't make those "beer barrel" filters

They were made, in England, by Aerial Facilities, now part of Cobham Wireless who don't even show a picture of them on the web site :-(

Fred
(ex-AFL)

Juliet Bravo · 30th Jan 2018, 12:14 pm

Thanks for all the info. It's very interesting, but I'm a complete novice at this. I'm having trouble at times being able to identify the difference between an unused flag pole and a lightning conductor!!

Jon_G4MDC · 30th Jan 2018, 4:23 pm

Spot on Fred, and here is the genuine article.
;-)

27th Jan 2018, 3:07 pm	#21
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: 153.35MHz and 153.025MHz? Some paging transmitter antennas were (are?) a long white pole usually about two or three metres in length and probably co-linear by design. Typically you would see more than one mounted on a tower, presumably one antenna for each channel in use. I think the transmitters were anything up to 100 watts output (as opposed to the usual 25 Watts from a PMR transmitter) and when fed into a co-linear this would provide quite a large signal. The average radio scanner receiver would typically suffer from breakthrough, if within a mile or two of the transmitter. I presume that this is due to a combination of sheer overload because of the transmit power, and the wide front ends typically used in scanners. I never worked on the national paging network but regularly saw them in shared site accommodation. We once had breakthrough on our main receivers from a co-sited paging network, but I think some notch filters sorted that if I remember correctly. Alan.

27th Jan 2018, 6:37 pm	#22
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: 153.35MHz and 153.025MHz? In times-past some of my repeaters shared masts with VHF pagers: the typical pager-antenna was a folded-dipole. On a "square" lattice mast there would usually be one such antenna on each corner, usually some way down from the top of the mast [cheaper to rent than at the top!] - depending on local/semi-local terrain sometimes you'd get two folded-dipoles on one corner if they needed to radiate more in a particular direction. Breakthrough was always a problem: plenty of the pager transmitters ran 500W and on a couple of occasions we needed to use 'barrel' cavity-filters [high-Q things that looked like a beer-barrel - inside the barrel there was a Silver-plated 1/4-wave element with an adjustable two-metal-plates capacitor to set the resonance] to keep the pager-RF out. This sort of thing: https://www.telewave.com/2017/07/31/...avity-filters/ On less-problematic sites we used multiple 1/4-wave and 1/2-wave coaxial 'stubs' made from LDF4-50A Heliax, which could easily give 70dB attenuation at the desired 'notch'.

30th Jan 2018, 12:14 pm	#24
Juliet Bravo Retired Dormant Member Join Date: Jan 2018 Location: Surrey, UK. Posts: 5	Re: 153.35MHz and 153.025MHz? Thanks for all the info. It's very interesting, but I'm a complete novice at this. I'm having trouble at times being able to identify the difference between an unused flag pole and a lightning conductor!!