Gb3hs on temporary antenna

Discussion in 'News' started by Clive G3GJA, 18 July 2016.

By Clive G3GJA on 18 July 2016 at 23:21
  1. Clive G3GJA

    Clive G3GJA Active Member
    Staff Member Website Admin Chairman

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    Currently you will be getting a much weaker signal from GB3HS. As of yesterday evening we a using a temporary antenna which is just a X-300 colinear at 10m AGL. This is part of a number of diagnostic tests we have been doing to find the cause of a mixing problem that interferes with the receiver of GB3HS. Prior to that we were using a Band III dipole cut for 195MHz at the top of the mast some 100 feet AGL that didn’t show the problem but again didn’t produce a good coverage.


    The antenna we should be using is a pair of stacked dipoles that are good for 140 to 175MHz. We swapped that onto a PMR system that was using the Band III dipole. However when connected a weak signal appears 3.5kHz away from the input frequency only when the transmitter is keyed; this doesn’t happen when the antenna is replaced with a good 50R load so the problem is in the antenna system somewhere.


    The difficulty we have is the that the tubular mast is mounted on the ridge of a large barn that’s about 40 feet up and its 100 feet wide so access to the aerials is very difficult. The site owner is a national PMR company and it doesn’t generate enough revenue from the site to warrant the investment in a new mast and aerials on their own so the repeater group is trying to raise some of the cost of getting a crane on-site for a day which looks to be in the region of £2000. I should say at this point that the PMR company has been very generous in allowing us full access, free of rent and electricity charges. They have even supplied 4 of the 6 cavities we use on the duplexer.


    The mixing problem may be related to the broadcast tower that’s about 400m away. Back in the late 90’s the repeater was on the opposite side of the same farm and there was a consistent problem caused by a mix where the third harmonic of Viking Radio on 96.9MHz running 30kW when mixed with our output frequency produced the input frequency. Written as an equation it looks like this: (96.9 x 3)-145.65=145.05MHz. We don’t think the current issue has the same cause as the equation causes the deviation to be 3 times that used on Band II i.e. 150kHz which is far too wide for a 12.5kHz receiver and consequently very easy to identify.


    Now, there’s just enough 88.5Hz near the input frequency to keep the transmitter keyed. On a spectrum analyser it’s about -122dBm on the receiver input; that’s 0.2microvolts. We can only attribute that to a bad metal to metal contact in the mast (which hasn’t been serviced since it was erected 40 years ago) or it’s related to some new multiplexes around 215MHz broadcasting DAB radio that were started just before Christmas last year, coincidentally, just when the interfere seemed to have started.


    So for now please bear with us while we trace the problem and try to raise money to get the mast changed. When done it will be climbable that will make its maintenance much easier and cheaper.


    73 Clive

    G3GJA / G8EQZ

    Keeper GB3HS
     
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    Last edited: 18 July 2016
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Discussion in 'News' started by Clive G3GJA, 18 July 2016.

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