Log periodic aerial elements length

[DIYspanner]

I bought one of these and looking at it, the elements seem odd in that they don't reduce in length uniformly towards the outer end so I'm wondering is this a properly made aerial?

Opinions welcome.

 

[opps]

Not my area of expertise but I have never seen a Yagi Uda like that before.

I think @Lucid will be able to advise you further.

 

[DIYspanner]

This is full length view of the elements.

 

[Lucid]

I should defer to someone with antenna building experience, but if I had to hazard a guess (and I emphasise a guess) then I would think this relates to the new shorter band for terrestrial now that the 700MHz clearance is complete.

The 'channels' are now 'squashed' into a smaller range, and so the chances of two muxes being just one or two RF channels apart is much higher. This could be to do with avoiding harmonics in the aerial, and so aiding channel separation.

Ericmark might be able to shed more light on this, or Rodders may too. I've kind of moved on to other areas.

 

[DIYspanner]

What intrigues me about the aerial I've bought is the elements near the outer end don't seem to be the correct lengths.

In photos of log periodic aerials I've seen online, the elements form a uniform taper whereas the one I have, they're a bit staggered in length.
Is this acceptable?

A better photo.

 

[endecotp]

I think you should ask them:

Contact

Unispectra

unispectra.com

I can imagine that in principle you might be able to adjust the frequency response in some way by adjusting the lengths - maybe the aim is to impove discrimination from e.g. mobile or something. I've not found anything by searching. It's the sort of thing you might find in a patent search.

Does it work OK?

 

[ericmark]

I tested the aerial on transmit, how much signal was reflected, or voltage standing wave ratio (VSWR), I have found that results are surprising, and I have transmitted and received 2 meter (144-146 Mhz) on a long wire, which in theory should not work.

Basic if it works it is OK, if it does not work it's not.

 

[Rodders53]

My former colleague John Riley of BBC R&D published the theory and plans to make one, a long time ago... albeit that'll have been for frequency channels 21-68 inclusive.

https://downloads.bbc.co.uk/rd/pubs/reports/1973-24.pdf

and an earlier report was

https://downloads.bbc.co.uk/rd/pubs/reports/1969-20.pdf

From a quick refresh read that parallel boom design is flawed wrt impedance matching (it may or may not matter?). But John also noted that they 'tweaked' the shorter elements' lengths to suit rather than use theoretical calculate lengths. Perhaps that's what this maker has done? The two longer elements at the front are wrapped in blue tape/paint on the makers website pictures but not in this real life sample.

Unispectra® “Raptor” High Gain Digital Logperiodic TV Aerial

About: “Raptor”: New generation LOG-periodic TV aerial. Newly redesigned and improved with very even level gain throughout all bandwidth. One of the most popular aerials in UK because of its simplicity of installation and very good wind resistance. Suitable for Freeview as well as Freeview HD...

unispectra.com

 

[DIYspanner]

I've tried the aerial in the loft which is where I plan to install it.

As a test, I laid it on the insulation and pointed it using a map reading compass to about 318°, the bearing quoted by freeview.co.uk

I did an autotune on the TV which found over 160 channels. I'm getting about 35 for signal strength.

I'll get a signal meter and will mount the aerial as high up as the roof apex will allow.

 

[foxhole]

Looks fine to me .

 

[Lucid]

I've tried the aerial in the loft which is where I plan to install it.

As a test, I laid it on the insulation and pointed it using a map reading compass to about 318°, the bearing quoted by freeview.co.uk

I did an autotune on the TV which found over 160 channels. I'm getting about 35 for signal strength.

I'll get a signal meter and will mount the aerial as high up as the roof apex will allow.

Lofts can be weird for aerial positioning. Be prepared to experiment.

Unless you can get hold of a very good meter, one with a display for the bit error rate per mux, then I'd recommend skipping any consumer meter and use a TV signal quality display instead. This isn't perfect. The TV is likely to display the quality after it's had a bit of a go at processing rather than the raw data. That's still better than the typical pocket meter though. They tend to measure average strength across all the muxes received.

I'd add to this one of the 0-20dB variable attenuators. This will give you the the chance to bring the signal down to a threshold level closer to where it's starting to fail. That's useful as it will make the TVs metering more sensitive to aerial positioning.

Don't forget to tilt the Log up at the front. You'll gain a couple of dB, and that's handy in a loft where the roof being wet can change the reception.

 

[Rodders53]

Loft can be full of 'standing wave' reflections...
So be prepared to move the aerial higher/lower., crab left/right and fore/aft in a cube of space with 1 metre sides... checking all 6 mux frequencies on the TV meter... all the while pointing directly at the transmitter.
Professionals use a spectrum analyser display to get them all roughly equal level.

Though as said TV meters are a variable feast. I use my trusty Humax HDR-Fox-T2 PVR for S/Q checks. So if you have a PVR try that, too. Sony used to show bit error rates on theirs both pre and post correction (v. useful).

Only separate meters worth having are £hundreds. https://cpc.farnell.com/labgear/s501/dvb-s2-t2-satterrestrial-meter/dp/IN09939 is one of the cheapest available.

Tilt up is a hangover from early DTT vehicle impulse interference issues I believe (but also advised by Justin of ATV aerials for water ingress / drainage issues in the front connection area when mounted externally). Unlikely to do any harm, though and may be essential in some locations with a mast/tower on a hill viewed from a valley.

 

[DIYspanner]

All comments noted - thanks!

Will post results is due course.

 

[Harry Bloomfield]

Tilt up is a hangover from early DTT vehicle impulse interference issues I believe (but also advised by Justin of ATV aerials for water ingress / drainage issues in the front connection area when mounted externally). Unlikely to do any harm, though and may be essential in some locations with a mast/tower on a hill viewed from a valley.

I seem to remember when reading the instruction for the instal of my new one some years ago, that it advised pointing it up a few degrees, for best reception.

 

[Rodders53]

Broadcasters don't bother with tilt unless they intend the beam to fire in that direction. CP UHF transmit has beam tilt to limit signals to the Continent and aims down a bit to the horizon rather than horizontal... 6 degrees rings a bell but it may be different.

Again, I suspect it's a hangover from initial DVB-T modulation method and early receivers being susceptible to impulsive interference from motor vehicles. Tilt up to reduce 'seeing' the foreground interference source (or use a roof ridge to block same). Same trick (use building to bloc) was used to avoid seeing foreground reflections in tidal fading conditions (though DTT may be more immune than analogue was).

"Best reception" = least interference not the strongest signal with DVB-T especially.

Logs have pretty wide polar diagrams so the tilt probably need to be a lot to reduce the received signal from the transmitter significantly... https://www.aerialsandtv.com/knowle...agrams#typical-aerial-polar-response-diagrams