What's this Antenna all about?

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
Post Reply
MiXiN
proppa neck!
proppa neck!
Posts: 508
Joined: Sun Sep 07, 2014 4:20 pm

What's this Antenna all about?

Post by MiXiN » Sun Aug 02, 2020 4:07 am

Not much to say really, but what's this all about?

How can you have -0.8DB gain less than an isotropic antenna or whatever it's called? Isn't an Isotropic antenna some form of baseline reference fake antenna with no gain?

Sorry for being a crayon eater, but does this thing serve a purpose or is it advertised as lossy & worthless? Surely a metal coathanger or a brass nipple would work as well? :D

Am I missing something? https://www.a****f.com/en/mixed-circula ... r-antenna/

User avatar
yellowbeard
no manz can test innit
no manz can test innit
Posts: 181
Joined: Wed Jul 08, 2015 5:40 am

Re: What's this Antenna all about?

Post by yellowbeard » Sun Aug 02, 2020 3:46 pm

It's a mixed polarity antenna, supposedly it gets into radios with their aerials up or down equally well. These are meant to be used in multiples - two of them is 0DBd, 4 is 3DBd - that's half the gain of the equivalent stack of vertical dipoles. So you have lost half your vertical radiated power into the horizontal plane, this means that at the absolute fringe of your range your signal is half as good on any one radio, but in close you are soaking all the radios no matter if their aerial is up (vertical) or down (horizontal). The guys on here don't like a mixed polarity antenna, but all the big boys use them without exception - they have 200Kw transmitters and thousand foot masts up a hill to stick 16 antennas on though. I think they have there uses - for me it'd be ideal, I got hills a couple of miles away on three sides of me here and the sea in the other - if you want to serve a small rural town it would also be the way to go.

MiXiN
proppa neck!
proppa neck!
Posts: 508
Joined: Sun Sep 07, 2014 4:20 pm

Re: What's this Antenna all about?

Post by MiXiN » Sun Aug 02, 2020 9:04 pm

Cheers for the info' mate. All interesting stuff.

Albert H
proppa neck!
proppa neck!
Posts: 1633
Joined: Tue Apr 05, 2016 1:23 am

Re: What's this Antenna all about?

Post by Albert H » Sun Aug 02, 2020 9:53 pm

Having done this lark for about 40 years, I can assure you that mixed polarisation and circular polarisation are just worthless fads for FM broadcasting. When I was engineering a major market FM flamethrower station in the western USA, they had a couple of huge towers with these "turnstile" aerials that they'd bought from Europe.

They'd been told that these were the canine's gonads as far as getting signal into the listeners' receivers. One site ran 850kW and the other ran 450kW. The signal was present almost everywhere, but suffered badly from multipath distortion, and was generally weak. Stereo was usually horrible.

After a week of driving 'round the service area with a test receiver, measuring field strength and multipath, and looking for "radio shadows" behind hills, big buildings and where the local topology screwed with the signal, I came up with a plan....

I got in touch with an aerial company in the UK, and had a heap of gamma-matched dipoles and phasing harnesses supplied. We took the station off the air for 16 hours overnight Sunday to Monday, and put the bigger site back on the air at 6am Monday morning. The lower powered site stayed off to allow us to do the aerial work during daylight hours (the rental on the floodlighting was really expensive!).

By 8am on the Monday, the switchboard had logged a couple of hundred calls from listeners commenting on how much better reception was, how much lower the distortion was, how it was better in their cars, and that they could cleanly receive it in places where it had previously been inaudible. By 9am, I had the MD of the station on the phone asking is we needed the secondary site at all, since the coverage from the primary was so exceptional!

I spent the next few days doing the same routes that I had done before, logging the field strength and the multipath. Field strength was up by around 30% in most areas, and multipath was acceptably low. The coverage was improved, with many of the "shadows" now gone, with weakened (but useable) signals there.

We sold the secondary site to another station about two months later, and re-filed with the FCC as a vertically polarised high power FM. This actually reduced the bills in several ways - not least when I had a dodgy PA valve at the site, and the power was reduced to ~500kW and we found that there was no significant reduction in field strength in the target area.....

Reading the original FCC Filings and the Operational Notes from the original "Chief Engineer", it became apparent that he had gone down the mixed polarisation route because it was trendy - "that's what all the mass-market stations are doing". There was no real reason for it whatsoever. >95% of the listeners used aerials with vertical polarisation (telescopic rods, car aerials and so on) and very few still used the old horizontally polarised, building-mounted aerials at all!
Last edited by Albert H on Sun Aug 02, 2020 10:03 pm, edited 1 time in total.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Albert H
proppa neck!
proppa neck!
Posts: 1633
Joined: Tue Apr 05, 2016 1:23 am

Re: What's this Antenna all about?

Post by Albert H » Sun Aug 02, 2020 10:00 pm

The "isotropic" aerial is a theoretical (not physically practical) aerial, used by some manufacturers as a yardstick to measure their antenna gain. A dipole has (notionally) 2.15dB gain above this theoretical, non-existent aerial! This allows charlatans to claim an extra 2.15 dB of gain!

I've always used a dipole as my reference. If properly matched and mounted, it provides a great baseline that can practically be used for tests. I always quote my gain figures as "dBd" - related to a real dipole!

The mixed polarisation aerials frequently give a significant loss in field strength (Watt for Watt), and the less scrupulous will measure this loss against an isotropic - it makes the aerial look less bad!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

MiXiN
proppa neck!
proppa neck!
Posts: 508
Joined: Sun Sep 07, 2014 4:20 pm

Re: What's this Antenna all about?

Post by MiXiN » Mon Aug 03, 2020 10:33 am

Again, all interesting reading - so many thanks Albert.

XXL
no manz can test innit
no manz can test innit
Posts: 128
Joined: Mon Sep 02, 2019 11:06 am

Re: What's this Antenna all about?

Post by XXL » Mon Aug 03, 2020 2:14 pm

Albert can you explain multipath distortion. I think my station is suffering from this. The weak clear carrier reaches double the distance of the strong areas. But when weak the music distorts and makes it unlistenable. The only way to fix it is to turn the music so far down that you need the receiver on full blast to hear it.

nrgkits.nz
Neckmin
Neckmin
Posts: 312
Joined: Fri Oct 17, 2014 10:35 am

Re: What's this Antenna all about?

Post by nrgkits.nz » Mon Aug 03, 2020 8:27 pm

Multipath distortion is where your receiver receives the same signal but from multiple paths, one path could be direct from the transmitter (line of sight) and the second path could be from a reflection off a building - this results in the phase being out and with a delay on the reflected signal. This will cause problems with stereo reception, creating noise. The same thing can happen with a rotten SWR as the reflected power is then retransmitted again with a delay and phase difference depending on your length of coax and losses involved.

mikroman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 16
Joined: Sun Apr 12, 2020 8:25 pm

Re: What's this Antenna all about?

Post by mikroman » Mon Aug 03, 2020 9:29 pm

not to be confused, it can also affect monophonic transmission. And it can be bad as inner grove distortion on LP (it sounds similar).

rigmo
no manz can test innit
no manz can test innit
Posts: 127
Joined: Wed Jan 30, 2019 9:35 pm

Re: What's this Antenna all about?

Post by rigmo » Mon Aug 03, 2020 10:00 pm

Albert H. absolutely right.. tnx

Albert H
proppa neck!
proppa neck!
Posts: 1633
Joined: Tue Apr 05, 2016 1:23 am

Re: What's this Antenna all about?

Post by Albert H » Wed Aug 05, 2020 11:17 pm

mikroman wrote:
Mon Aug 03, 2020 9:29 pm
not to be confused, it can also affect monophonic transmission. And it can be bad as inner grove distortion on LP (it sounds similar).
Multipath can really sound horrible - even in mono. There's no real way to eliminate it except by using a higher transmit site (so that the direct signal flattens the reflections by at least 10dB).

FM has this wonderful property that's called "capture effect" - it's somewhat receiver dependant - as long as there's a reasonable margin in signal strengths, the receiver will only demodulate the stronger one. This property is used to all re-use of frequencies by distant stations. The measure of how well the receiver separates two signals on the same frequency is called the "capture ratio" and is a quoted parameter on better quality receivers.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

mikroman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 16
Joined: Sun Apr 12, 2020 8:25 pm

Re: What's this Antenna all about?

Post by mikroman » Thu Aug 06, 2020 6:02 pm

Thanks Albert. You always have an interesting topic to add :)

Post Reply