Taxi mag mount antenna for scanning

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Taxi mag mount antenna for scanning

Postby Southwales » Sun Jun 24, 2018 12:39 am

Hi, Could this taxi antenna below be used on a vehicle for scanning, rather than the diamond 2m/70cm antenna I am using now, hoping this more basic taxi mag may be slightly better at mil air and CB frequencies compared to the diamond, as hoping it is basically just a piece of wire In the air. At the moment I am using my diamond antenna to also listen CB fm and air band etc but feel the diamond is to dedicated to 2m/70cm.

https://www.ebay.co.uk/itm/PL259-VHF-UH ... Swh79a9D2f

.....
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Re: Taxi mag mount antenna for scanning

Postby Minus1 » Sun Jun 24, 2018 7:16 am

CB is a LONG way from Mil Air, you are not going to find an antenna that is optimized for both.
You might be able to tune this to (say) 300 MHz, but it's not likely to improve CB reception over any random bit of wire.
You really need separate antennas designed for each band.
I reserve the right to ignore people who have made no attempt to the read the manual, and expect others to do it for them.
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Re: Taxi mag mount antenna for scanning

Postby m0lsx » Sun Jun 24, 2018 9:03 am

As per minus one.
Really no single commercial mobile antenna is going to be designed to do what you want it to do. You will see claims of a band coverage, but any piece of wire, could legitimately be said to be capable of receiving from Ultra low frequencies in the single KHz range with a wave length of many Kilometres to microwave's of millimetres long in the Ghz range.
The problem is that there is a difference between being legitimately able to make such a claim of capability & the antenna being of any real use across the whole of that range.
One option that may improve reception on both the top end of HF (CB) & on UHF (Mil Air) is an amateur radio antenna designed for a 4 band FM radio that covers 10 meters, (29 MHz.) 6 meters, (50 MHz.) 2 meters, (145Mhz) & 433Mhz (70cm.) So from just above CB to above & below Mil Air. But any antenna has a bandwidth on any band & in general the wider the band the less effective an antenna is. Although a properly designed Discone or Bow Tie antenna are both effective & wideband. So almost any antenna even a few MHz away from it's designed operating frequency is going to be a compromise.
As an example of bandwidth, my Diamond CP-6, has an operating band of only a few KHz on 80 meters (3.8MHz.) & needs a very good Antenna Matching Unit to allow the use of the whole of the SSB section of 80 meters. Even many single band antennas are not very effective, even on their designed for band. Antenna design, especially multi band single vertical designs, are always going to be about compromise.
https://www.ebay.co.uk/itm/CR8900-10-6-2-70-mobile-antenna-UK-Supplier/192573980642?hash=item2cd64dbfe2:g:4AAAAOSwWWxY-ZWW
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Re: Taxi mag mount antenna for scanning

Postby G4RMT » Sun Jun 24, 2018 9:53 am

My Van has a number of antennas for things I do for work - the most useful for a scanner is my marine band. I tad too short for 2m, a tad too long for business band VHF, but it actually works fine on transmit on both - acceptable VSWR. It works on 70cm for transmit too - as a ¾ wave, and I can use it to access my UHF business repeater from around 8 miles reliably. The recent air show showed it was a good compromise for reception on VHF and UHF hairband. There is a UHF ¼ wave on the van too, but I rarely use that one as the difference between the 1/4W VHF working on UHF and the1/4W cut for UHF is minimal.

None of the antennas work very well at all on CB, the few times I've tried it, and they're pretty poor on 4m too. In fact, my van radio is pretty deaf so I thought maybe using one for radio 2 would improve things ion the fitted van one - it didn't!

That Ebay link is a bit weird - a taxi antenna that is 138-147??? I doubt that title is correct - but later in the text it says the Tim up to 480MHz, so they're clearly just dim.

I would have no hesitation buying one of these for a vehicle if I wanted to use a scanner - I would trim it to 157MHz, and be happy. In fact, I just bought one for £15 as it could be quite useful, and frankly it's dirt cheap. If you want, I'll report back when it comes - but it's a 1/4W, so the only unknown is the magnet!
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Re: Taxi mag mount antenna for scanning

Postby Southwales » Sun Jun 24, 2018 4:49 pm

Hi, G4RMT, do you mean you have purchased the one I linked to, if so yes would really like it if you could let me know how it goes and what length you decided to trim it to.

Basically my uniden 125 scanner has the first 4 banks full and this includes CB fm, PMR licence free, mil air, ofcom business frequencies, and the baofeng 888 frequencies etc so a good range, but with living out in mid Wales and the airwaves being fairly quiet I want to scan through all the frequencies at once in the hope of having something to listen to during my commute, If say I just decided to scan CB fm with the correct antenna then on that journey I may get nothing and could then be missing transmissions on PMR, hence I want to spread my net wide in the hope of catching as much as possible or at least a few different things to listen to on my 1 hour plus commute. I understand the compromise of using an antenna for a certain band is going to be a compromise on another, but I want to try and find an antenna that is the best compromise for all. That is why I was thinking this antenna basically being nothing fancy just a length of coax, magnet base and 45cm of wire pointing upwards could this be middle ground? Don't get me wrong If I was scanning why stationery on top of a hill then maybe I could swop antennas but I want to scan through my stored frequencies why on the move. At home it is much easier as I can use my bi-cone and that seems a good compromise pulling in anything within a few miles including CB fm. PMR licence free and even ISS space station. If I tried to put something that looked even a bit like a bi-cone on top of the car my wife and kids would not be impressed LOL. My other thought is maybe I had the best compromise with my original £4 mini Nagoya magmount that did seem to be reasonably okay at recieving CB FM and PMR etc but wondering if this taxi mag could be slightly better as it has thicker rg58 coax and about 17cm taller whip over the cheaper Nagoya mini mag with the thin rg213 coax.
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Re: Taxi mag mount antenna for scanning

Postby G4RMT » Mon Jun 25, 2018 10:01 pm

Yep - I used your link and ordered one. I'll let you know what it's like when it arrives.
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Re: Taxi mag mount antenna for scanning

Postby Southwales » Wed Jun 27, 2018 2:09 pm

G4RMT wrote:Yep - I used your link and ordered one. I'll let you know what it's like when it arrives.


Thanks, Do you think the antenna will need shortening?

May be a silly question but is it fair to say as a general rule the higher up the frequency then the shorter an antenna would ideally be?
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Re: Taxi mag mount antenna for scanning

Postby m0lsx » Wed Jun 27, 2018 3:04 pm

Southwales wrote: May be a silly question but is it fair to say as a general rule the higher up the frequency then the shorter an antenna would ideally be?


Yes the higher the frequency the shorter the wave length.
300 divided by MHz gives you a wavelength & without complicating matters too much. The length of an antenna is slightly shorter than the radio waves calculated length.

Some rough ideas.
300 divided by 30 mhz = 10 meters.
300 divided by 144 mhz = 2 meters & a few centimetres.
300 divided by 433 mhz= 69 centimetres.
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Re: Taxi mag mount antenna for scanning

Postby G4RMT » Wed Jun 27, 2018 6:14 pm

It's arrived - decent quality with a ⅜"screw in base, strong magnet and soldered PL259 connector.

As it comes - the whip is 50.1cm, from the base when screwed in, and resonates on just over 140MHz. I've trimmed mine back to 45cm, so it's fine on marine band - and then OK on the ham bands and business band. Seems to be identical performance wise to my other antennas on the van.

For £15 - a great deal.

Paul
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Re: Taxi mag mount antenna for scanning

Postby Southwales » Thu Jun 28, 2018 12:39 am

G4RMT wrote:It's arrived - decent quality with a ⅜"screw in base, strong magnet and soldered PL259 connector.

As it comes - the whip is 50.1cm, from the base when screwed in, and resonates on just over 140MHz. I've trimmed mine back to 45cm, so it's fine on marine band - and then OK on the ham bands and business band. Seems to be identical performance wise to my other antennas on the van.

For £15 - a great deal.

Paul


Hi, Ordered mine earlier today, is that trimmed 45cm from the top of the magnetic base, So you removed 5cm is that correct? Is it likely to be any good on pmr 446 at this length?
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