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Labgear Tri-Boom Mast Digital Aerial (or similar)



 
 
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  #22  
Old September 27th 18, 07:46 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On 27/09/2018 18:31, charles wrote:
In article ,
Bill Wright wrote:

[Snip]

Log periodics aren't just used for UHF TV. Some DAB aerials are logs,
and logs are widely used for VHF TV, HF (short wave), and for direction
finding.


There used to be a big HF one on the Chinese Embassy in London. The World
Radio & TV Handbook once carried an advert for an HF Log where the elements
looked over a foot in diameter


I used to drive past that quite often.

Bill
  #23  
Old September 27th 18, 07:50 PM posted to uk.tech.digital-tv
Woody[_4_]
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Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On Thu 27/09/2018 18:31, charles wrote:
In article ,
Bill Wright wrote:

[Snip]

Log periodics aren't just used for UHF TV. Some DAB aerials are logs,
and logs are widely used for VHF TV, HF (short wave), and for direction
finding.


There used to be a big HF one on the Chinese Embassy in London. The World
Radio & TV Handbook once carried an advert for an HF Log where the elements
looked over a foot in diameter


You should wander up here and look at the HF LPs at the former RNAS
Forest Moor adjacent to the NSA spy base at RAF Menwith Hill. They are
HUGE!!

https://tinyurl.com/yaq5t8ch
Look to the left of the lane on the right of the VT Comms sign.

--
Woody

harrogate three at ntlworld dot com
  #24  
Old September 27th 18, 08:11 PM posted to uk.tech.digital-tv
Jeff Layman[_2_]
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Posts: 817
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On 27/09/18 15:04, Bill Wright wrote:
On 27/09/2018 13:42, Jeff Layman wrote:
On 27/09/18 04:08, Bill Wright wrote:

Everything is OK for digital. Forget that.
https://cpc.farnell.com/blake-uk/bla...ial/dp/AP03352

Or ring ATV Sheffield, ask for Justin, mention my name.
http://aerialsandtv.com/


Looks good.* But how do you tell the difference (if there is any)
between different log periodics? The following are all similar prices,
although somewhat more expensive than the Blake. Is there any practical
difference if they are for loft mounting, rather than being outside?

https://www.amazon.co.uk/AERIAL-LOG-PERODIC-36-ELEMENT-TYPE/dp/B01L62DT7G/ref=sr_1_4?ie=UTF8&qid=1538046869&sr=8-4&keywords=log+periodic+aerial


https://www.amazon.co.uk/36-Element-Log-Periodic-Aerial-Pre-Wired-F-Connector/dp/B016MWH5O8/ref=sr_1_5?ie=UTF8&qid=1538046869&sr=8-5&keywords=log+periodic+aerial


And this is perhaps smaller, but seems to get good reviews (if you
believe Amazon reviews...):
https://www.amazon.co.uk/Raptor-LTE-Periodic-Digital-Antenna-silver/dp/B016Y90NLQ/ref=sr_1_6?ie=UTF8&qid=1538046869&sr=8-6&keywords=log+periodic+aerial


They are all OK, but the short one has slightly less gain.

Log periodic aerials have two parallel booms, usually close together.
These booms together form a transmission line. A series of half wave
dipoles are attached to the booms, each dipole having one quarter wave
section fixed to Boom A and the other to Boom B. Each dipole (except two
or three; see later) are tuned to a frequency within the range required
for reception or transmission, and are fixed in order of resonant
frequency along the two booms. Each dipole is connected in opposite
phase to its neighbours. The two booms are connected together
electrically at the low frequency end of the aerial. The feeder cable is
connected across the two booms at the high frequency end. The feeder
often runs inside one of the booms to its low frequency end, where the
aerial is fixed to the mast. Running the feeder inside the boom that is
connected to the feeder’s screen seems to reduce the need for
balance/unbalance conversion. The dimensions and spacing of the two
booms is designed to produce an impedance of approximately 75Ω. For
these reasons log periodics don’t seem to need a balun.

And the extra two or three dipoles? One is just slightly longer than
would be resonant at the lowest operating frequency and then there’s
another one or two that are shorter than would be resonant at the
highest operating frequency.

Each element of a log periodic is longer or shorter than its neighbour
by a fixed ratio, and the inter-element spacing also increases or
decreases by a fixed ratio. The inter-element spacing is generally a
quarter wavelength of the adjacent dipole’s resonant frequency, or a bit
less.

The smaller the ratio of the lengths of adjacent dipoles the higher the
gain. A large ratio will lower the gain but increase the bandwidth for a
given number of dipoles. Just as the yagi is inherently narrowband the
log is inherently wideband. Almost all logs sold in the UK for UHF TV
reception are wideband in that they cover all the channels used for TV
broadcasting. However Vision Products have a ten-dipole product that
covers Channel Group A only. The gain as you might expect is slightly
better than that of a ten-dipole log designed for all channels.

For a given frequency only a small zone along the boom of the log
periodic is active. The zone is centred on the dipoles that are at or
near resonance at that frequency. Consider two adjacent dipoles,
somewhere about half way along the boom. The incoming signal is in the
middle of the frequency range of the aerial, so the two dipoles are
approximately resonant and are therefore in the active zone. Feeder
reversal and the distance between the two elements gives a phase shift
of 360° between them. In other words, for a signal coming from the
‘front’ of the aerial (only) the signal on the two dipoles is additive
if it is collected from the front of the boom-cum-transmission line.
This is why log periodics are directional, and why the feed-point is at
the sharp end. It’s possible for three or even four dipoles to work
together in this way. A smaller ratio between the lengths of adjacent
dipoles means more dipoles are near resonance for a particular
frequency, which is why the gain is higher for such an aerial. There are
two things that prevent the dipoles that are shorter and longer than the
resonant ones from playing much of a part in reception. One is that they
aren’t resonant and the other is that the spacing between them does not
produce the 360° of phase shift that is essential for the aerial to have
gain and directivity. The spacing between the directors on a yagi can be
varied widely (and often is to help feeder matching) but with a log
periodic the spacing has to be much more closely defined, and has to
take into account the velocity factor of the twin booms. (Velocity
factor is the speed of radio transmission in anything other than free
space; for instance feeder cable or aerial elements.)

If only a few dipoles are active, why does a typical log periodic have
as many as twenty? It’s because the more dipoles there are (for a given
ratio between the lengths of adjacent dipoles) the greater the range of
frequencies the aerial will receive. This is simply because the range of
the lengths of the dipoles is greater.

The active zone shifts with frequency, and you can easily find the
active zone of a log periodic for a given frequency. Monitor the signal
output of an aerial at that frequency using a meter or spectrum analyser
and get hold of one end of each dipole in turn. This will de-tune that
dipole. When the signal level drops you’re holding one of the active
dipoles.

If a log periodic is compared with a yagi of the same size it’s very
much a case of horses for courses. The yagi will have much better gain
if it’s a grouped one, but if it’s wideband its gain at the lower end of
the band might be no better or even a bit worse than that of the log
periodic. The main advantages of the log periodic are that the response
is very flat right across the band and that the directional
characteristics are also maintained at all frequencies. This is where
the wideband yagi falls down.

The log will have a far better front-to-back ratio than the equivalent
yagi. This means that it is better at rejecting signals from the rear.
However the front lobe (the ‘acceptance angle’) of a log is much wider
than that of a grouped yagi, or of a wideband yagi when used on the
higher channels. This means that if it’s necessary to receive from two
transmission sites that aren’t co-sited but are within fifteen or twenty
degrees of each other the log can work well.

When it comes to rejecting unwanted signals from the side there isn’t
much in it when the polarisation is horizontal, but when it’s vertical
the log wins hands down.

Log periodics aren’t just used for UHF TV. Some DAB aerials are logs,
and logs are widely used for VHF TV, HF (short wave), and for direction
finding.

Bill


Thanks for the detailed explanation. I've saved it for future reference
the next time the frequencies are messed with.

According to the digitaltv coverage checker I'm in a "good" reception
area for all transmissions (from Rowridge), so any of those log periodic
aerials should be ok

--

Jeff
  #25  
Old September 27th 18, 10:33 PM posted to uk.tech.digital-tv
Vir Campestris
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Posts: 496
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On 27/09/2018 15:04, Bill Wright wrote:
Each element of a log periodic is longer or shorter than its neighbour
by a fixed ratio, and the inter-element spacing also increases or
decreases by a fixed ratio.


Which is the source of the name. Logarithmic.

Andy
  #26  
Old September 27th 18, 11:32 PM posted to uk.tech.digital-tv
tony sayer
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Posts: 4,065
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

In article , Jeff Layman
scribeth thus
On 27/09/18 04:08, Bill Wright wrote:

Everything is OK for digital. Forget that.
https://cpc.farnell.com/blake-uk/bla...-log-periodic-

aerial/dp/AP03352
Or ring ATV Sheffield, ask for Justin, mention my name.
http://aerialsandtv.com/


Looks good. But how do you tell the difference (if there is any)
between different log periodics? The following are all similar prices,
although somewhat more expensive than the Blake. Is there any practical
difference if they are for loft mounting, rather than being outside?

https://www.amazon.co.uk/AERIAL-LOG-...1L62DT7G/ref=s
r_1_4?ie=UTF8&qid=1538046869&sr=8-4&keywords=log+periodic+aerial

https://www.amazon.co.uk/36-Element-...-F-Connector/d
p/B016MWH5O8/ref=sr_1_5?ie=UTF8&qid=1538046869&sr=8-5&keywords=log+periodic+aeri
al

And this is perhaps smaller, but seems to get good reviews (if you
believe Amazon reviews...):
https://www.amazon.co.uk/Raptor-LTE-...er/dp/B016Y90N
LQ/ref=sr_1_6?ie=UTF8&qid=1538046869&sr=8-6&keywords=log+periodic+aerial





Just fixed up one of these yesterday to replace a 40 odd year old Yagi
works fine and quite well built too!..


https://cpc.farnell.com/antiference/...og-periodic-f-
conn/dp/AP01931
--
Tony Sayer



  #27  
Old September 27th 18, 11:48 PM posted to uk.tech.digital-tv
[email protected]
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Posts: 1,270
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On Thu, 27 Sep 2018 09:26:33 -0700 (PDT),
wrote:

On Wednesday, September 26, 2018 at 7:38:05 PM UTC+1, wrote:
First post - I have a TV aerial that looks the same shape as the Labgear Tri-Boom Mast Digital Aerial -
https://www.screwfix.com/p/labgear-t...l-aerial/60215

The topmost part of the rear reflector (?) wobbles by about 25mm as it appears to rotate at its connection point to the main horizontal component. The aerial feeds a splitter/distribution box and we are getting an intermittent broken picture on one of the two TVs that it supplies.

Is the loose part likely to be the cause of the problem, can it be taped/cable tied still or does the aerial need to be replaced?

Many thanks

Martin


Wow - many thanks for the variety of helpful responses. Following Bill's recommendation Ive been in touch with ATV today - I've photographed the entire installation and sent the images along with questions for Justin tomorrow with a commitment to buy any equipment they suggest to solve the immediate problem and bring the installation to the best possible situation. The knowledge that ATV (and companies like them) provide is essential for people like me, Im keen to support them even though B&Q etc may be cheaper.
Martin


Just a reminder of something that was mentioned earlier.
If the existing aerial connection box has allowed water into the coax,
that may need replacing too. Check that there is no water around the
connections inside the box, and that the copper doesn't look corroded.
  #28  
Old September 28th 18, 12:40 AM posted to uk.tech.digital-tv
Woody[_4_]
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Posts: 1,968
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On Thu 27/09/2018 22:32, tony sayer wrote:
https://cpc.farnell.com/antiference/...og-periodic-f-
conn/dp/AP01931


I'm sorry to have to disagree in part Tony but there are two things in
the CPC advert that rancour. It says 'highly directional' which we know
a LP isn't (per BW's contribution earlier) and in this country what is
the point of any aerial that can only be mounted horizontally in its
basic format?

Normally CPC are pretty good at what they sell but I think they've
slipped up with this one!

--
Woody

harrogate three at ntlworld dot com
  #29  
Old September 28th 18, 05:21 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 3,079
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On 27/09/2018 23:40, Woody wrote:
On Thu 27/09/2018 22:32, tony sayer wrote:
https://cpc.farnell.com/antiference/...og-periodic-f-
conn/dp/AP01931


I'm sorry to have to disagree in part Tony but there are two things in
the CPC advert that rancour. It says 'highly directional' which we know
a LP isn't (per BW's contribution earlier)


Logs have a reputation for being highly directional because the
front-to-back and front-to-side ratios are very high. However the main
front lobe is actually very wide, so the aerial offers little protection
against interference coming from a quite a few degrees either side of
the tx direction.

The reputation for good directivity comes largely from the analogue
days, because it's unusual for a reflected signal (the cause of
ghosting) to come from the front quadrant. (It can happen though,
especially in cities with high rise flat-sided buildings. It was for
instance a major problem at a lot of the sites I worked at in central
London.)

The wide forward lobe shouldn't put anyone off using a log periodic. To
generalise wildly, transmission stations are often on high ground, which
means that co-channel interference from further away in the same
direction is unlikely to make the trip over the hill.

If the directivity of a log periodic isn't enough it helps a lot to
mount two side by side and combine the outputs.

and in this country what is
the point of any aerial that can only be mounted horizontally in its
basic format?


It's odd that you have to pay extra for a VP clamp. Most logs are
designed so they can be installed both ways. I can only think someone
was unaware the the UK uses VP!

Bill
  #30  
Old September 28th 18, 09:12 AM posted to uk.tech.digital-tv
Jeff Layman[_2_]
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Posts: 817
Default Labgear Tri-Boom Mast Digital Aerial (or similar)

On 28/09/18 04:21, Bill Wright wrote:
On 27/09/2018 23:40, Woody wrote:
On Thu 27/09/2018 22:32, tony sayer wrote:
https://cpc.farnell.com/antiference/...og-periodic-f-
conn/dp/AP01931


I'm sorry to have to disagree in part Tony but there are two things in
the CPC advert that rancour. It says 'highly directional' which we know
a LP isn't (per BW's contribution earlier)


Logs have a reputation for being highly directional because the
front-to-back and front-to-side ratios are very high. However the main
front lobe is actually very wide, so the aerial offers little protection
against interference coming from a quite a few degrees either side of
the tx direction.

The reputation for good directivity comes largely from the analogue
days, because it's unusual for a reflected signal (the cause of
ghosting) to come from the front quadrant. (It can happen though,
especially in cities with high rise flat-sided buildings. It was for
instance a major problem at a lot of the sites I worked at in central
London.)

The wide forward lobe shouldn't put anyone off using a log periodic. To
generalise wildly, transmission stations are often on high ground, which
means that co-channel interference from further away in the same
direction is unlikely to make the trip over the hill.

If the directivity of a log periodic isn't enough it helps a lot to
mount two side by side and combine the outputs.

and in this country what is
the point of any aerial that can only be mounted horizontally in its
basic format?


It's odd that you have to pay extra for a VP clamp. Most logs are
designed so they can be installed both ways. I can only think someone
was unaware the the UK uses VP!


What main transmitters other than Rowridge use HP? I could look it up
but it's quicker to use this newsgroup!

--

Jeff
 




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