Quick antenna question for the radio buffs....

[Bobbybass]

I have just bought a new radio which comes with a 'flat surface' mounted antenna. I asked about a magentic base...as I didn't want to bolt it onto the boat all the time...but they don't do one.

There are several suppliers of CB mag bases on Ebay....Just the bases...wired complete with RG58 cable and PL259 plugs.( I KNOW it will damage the radio to use a CB mast....don't worry)

My question is...would it be alright to use one of these mag bases...adapted to screw my new antenna into the base...as long as I don't lengthen the operating length of the aerial?

 

Thanks.

 

Bob

[carlt]

Using a CB aerial won't damage a radio receiver.

 

It will only damage a transmitter with an incorrect wavelength.

[Bobbybass]

Using a CB aerial won't damage a radio receiver.

 

It will only damage a transmitter with an incorrect wavelength.

 

Its a marine transmitter...not receiver...

 

25 watt output...

 

Thank you...but that wasn't my question.

[carlt]

Its a marine transmitter...not receiver...

 

25 watt output...

 

Thank you...but that wasn't my question.

You're welcome.

 

Why don't you just buy a marine antenna with a mag mount?

 

Like this one...

 

Clicky

[Bobbybass]

You're welcome.

 

Why don't you just buy a marine antenna with a mag mount?

 

Like this one...

 

Clicky

Because the antenna came with the radio... a CB mag mount (without antenna) is about £10. A purpose made marine mag mount/antenna is a lot more than that...so I was enquiring about the cheaper option as I have the antenna.

Edited January 3, 2012 by Bobbybass

[carlt]

Then if you can find a mag mount, with the correct screw fitting, then it will be just fine.

[Bobbybass]

Then if you can find a mag mount, with the correct screw fitting, then it will be just fine.

 

Great...thanks.

[Nickhlx]

Because the antenna came with the radio... a CB mag mount (without antenna) is about £10. A purpose made marine mag mount/antenna is a lot more than that...so I was enquiring about the cheaper option as I have the antenna.

 

I can't really imagine the sort of antenna it comes with, if it's not a mag mount type, where the proximity of the (metal) roof (that it would normally be put on) forms part of the antenna system. I suppose it could be an isopole on some sort of wooden tripod, or a J-pole (J-Stick) or a "Slim-Jim", which would each be approx 50" long ( and ground plane independent). The latter two I use extensively at VHF and UHF as they are simple, inexpensive and easy to make, and excellent for omni-directional range in a non-hilly environment as they have very low angle of radiation, but at 50" or so in length at 156 MHz I wouldn't expect them to be practical to ship with the transceiver.

 

From your description it appears to have come with a ( assuming suitable for the frequency) aerial that can sit on e.g a (metal) roof but is not magnetic. In the instructions for the use of the included aerial, does it not give any guidance notes on how this aerial should or should not be used ? Can you give us a picture of it, or is it not very meaningful as to what it might be if it is enclosed in a white fibreglass tube ? I suppose it could also be two helicals, back to back and centre-fed, so pretty inefficient....

 

For receiving, anything would "work" in as much as it wouldn't hurt/damage the receiver, but for TX you would need a well-matched antenna or it would damage the output stage.

 

To answer your query, I think you could use a CB mag mount base if it was not part of the aerial system, and used purely to hold the supplied aerial. You might get better performance though if you could cut a rod / piece of stiff wire to about 47 cms long and use it as a proper magmount - You would need to use a SWR meter to optimise the length though...

 

Nick

Edited January 3, 2012 by Nickhlx

[Bobbybass]

I can't really imagine the sort of antenna it comes with, if it's not a mag mount type, where the proximity of the (metal) roof (that it would normally be put on) forms part of the antenna system. I suppose it could be an isopole on some sort of wooden tripod, or a J-pole (J-Stick) or a "Slim-Jim", which would each be approx 50" long ( and ground plane independent). The latter two I use extensively at VHF and UHF as they are simple, inexpensive and easy to make, and excellent for omni-directional range in a non-hilly environment as they have very low angle of radiation, but at 50" or so in length at 156 MHz I wouldn't expect them to be practical to ship with the transceiver.

 

From your description it appears to have come with a ( assuming suitable for the frequency) aerial that can sit on e.g a (metal) roof but is not magnetic. In the instructions for the use of the included aerial, does it not give any guidance notes on how this aerial should or should not be used ? Can you give us a picture of it, or is it not very meaningful as to what it might be if it is enclosed in a white fibreglass tube ? I suppose it could also be two helicals, back to back and centre-fed, so pretty inefficient....

 

For receiving, anything would "work" in as much as it wouldn't hurt/damage the receiver, but for TX you would need a well-matched antenna or it would damage the output stage.

 

To answer your query, I think you could use a CB mag mount base if it was not part of the aerial system, and used purely to hold the supplied aerial. You might get better performance though if you could cut a rod / piece of stiff wire to about 47 cms long and use it as a proper magmount - You would need to use a SWR meter to optimise the length though...

 

Nick

 

Its one metre long...white covered...has a small capsule at the bottom...and then just attaches to an bracket...for bolting to whatever takes your fancy. No suggestion of a ground plane of any kind...specified up to 100 watts of output transmission power.

 

Actually...this is it...although mine does not have this bracket..mine just has a plate for flat surfaces...hence...I thought I could screw it into a CB mag base that comes with the RG58 and PL259....No..??

I have an old CB SWR meter...will that work ?

 

http://www.ebay.co.uk/itm/MARINE-300-3dBd-VHF-156MHz-FIBREGLASS-BOAT-ANTENNA-/350497043679?pt=UK_CarsParts_Vehicles_BoatEquipment_Accessories_SM&hash=item519b4074df

Edited January 3, 2012 by Bobbybass

[carlt]

Its one metre long...white covered...has a small capsule at the bottom...and then just attaches to an bracket...for bolting to whatever takes your fancy. No suggestion of a ground plane of any kind...specified up to 100 watts of output transmission power.

 

Your boat is the ground plane, just as it would be with a mag mount.

 

The one in your link looks like it has a fairly standard thread that will screw into a variety of mounts.

 

Take it along to a CB shop and they should have the correct mag mount for it.

 

CB swr meters tend to be a bit rubbish but even if yours is good I'm pretty sure it will be useless on the VHF wavelengths.

[alan_fincher]

The link you have shown for your antenna indicates it is terminated by an SO239, so is expecting to have a PL259 to connect it to the set.

 

In my experience with mag mounts, (CB and some amateur radio types), they effectively often have an SO239 as what the antenna screws to, expecting the antenna itself to have effectively a PL259 at it's base.

 

So the problem you face is that you are likely to be trying to mate two SO239 like sockets together, I would have thought.

 

It is just possible that some kind of PL259 to PL259 adaptor may work, giving a male at each end that will plug into your two female parts, (ooh, err, missus!).

 

Something like this.....

 

 

But I'm not confident, and it certainly wouldn't be very watertight, unless bound over with something.

 

Obviously you want to use your antenna, but another good alternative is to cut down a 2 metre whip antenna designed for amateur aadio - much cheaper than many marine antennas, and just as efficient once trimmed for the somewhat higher frequencies.

 

But 27MHZ CB antenna could not be easily so adapted, of course - they have a loading coil, often as part of the base that screws them to a mount.

 

Most cheap SWR meter designed to work around 27MHZ will in my view be pants at marine VHF frequencies, although one designed for 2 metre Amateur Band would be fine.

 

If the "Burton" in your location is Burton-on-Trent then you are unfortunately too far away for me to easily lend you one.

 

Alternatively, ould you not use any old mag mount base, but instead of trying to adapt it to screw your antenna to "electrically", simply come up with a way of making it hold your antenna (and existing mounting bracket), in its entirety so that the SO239 is still available for you to connect to the transmitter via a basic lead terminated in a PL259 ? May not be pretty, but should work OK, I would have thought ?

[Nickhlx]

Your info on the length and the lump seems to have confirmed it for me....

 

I think it is an end-fed 1/2 wave aerial which is also ground plane independent - i.e. it does not rely on the existence of a metal roof to be the missing half of the aerial system. In fact, the close proximity is likely to be detrimental as it will skew the optimum signal lobe upwards, away from the desired horizontal position that it will give in free space. The "lump" at the bottom is the matching coil / network to transform the approximately 50 ohm feed impedance to the high impedance feedpoint at the end of the half wave radiating element...

 

As regards the SWR meter for CB frequencies ( I assume 27 MHz not 446 ?) it's unlikely it will work accurately. One I have for amateur use is "rated" for up to 150 MHz and it does move its needles, but I wouldn't think its at all accurate. However, it may well be OK for relative readings, i.e. to trim a 1/4 wave whip down from say 50 cms to about the wanted 47 cms for 156 MHz ( a few mm at a time and use low power, say 1 watt, but don't stand near it whilst testing (else your body mass will de-tune it).

 

As others have said if your aerial cable terminates in a PL-259 PLug, and the aerial connection on the rig is not a fixed SOcket ( SO-239) but also a short cable terminated in another PL-259, you will need the female-female coupler ( SO-239 to SO-239 back to back). Any Ham radio shop will have them or do them mail order (e.g. Nevada on (02392) 313090, Martin Lynch and Sons (0345) 2300 599 or Waters and Stanton on (01702) 206835, and probably Maplin, or there's always E-bay...

 

Nick

 

 

Edited January 3, 2012 by Nickhlx

[by'eck]

I have just bought a new radio which comes with a 'flat surface' mounted antenna. I asked about a magentic base...as I didn't want to bolt it onto the boat all the time...but they don't do one.

There are several suppliers of CB mag bases on Ebay....Just the bases...wired complete with RG58 cable and PL259 plugs.( I KNOW it will damage the radio to use a CB mast....don't worry)

My question is...would it be alright to use one of these mag bases...adapted to screw my new antenna into the base...as long as I don't lengthen the operating length of the aerial?

 

Thanks.

 

Bob

 

So from comments I understand you refer to a marine transceiver (transmitter/receiver) working in the 150-160MHz range. The transmit performance of this will be seriously impaired if you don't use a dedicated matching aerial. A large proportion of the signal will be bounced back (poor SWR) when the signal meets aerial resulting in substantially reduced propagation. You can buy an SWR meter for a moderate price to check this.

 

If you don't want to permanently mount a matching aerial, why don't you source a magnetic mount with adjustable pivot. The correct aerial can then be attached to this & allow folding down for bridges etc. One of the best examples below, which having a base loading coil doesn't rely on the roof for ground plane.

 

 

RG58 cable with PL259 connectors each end will be fine for the length of cable run I imagine you will need. Whatever you use its important it has a 50Ω impedance.

 

m0dsu

Edited January 3, 2012 by richardhula

[alan_fincher]

One of the best examples below, which having a base loading coil doesn't rely on the roof for ground plane.

 

I may be missing something, but doesn't OP already have a dedicated antenna that can be used in exactly that way, and is not relying on the boat for a ground plane ?

 

I guess my only concern, (and I'll admit that I am more out of my comfort zone here!), is that I would guess antennas like both he and you have pictured usually get strapped to the top of GRP boats, not massive steel ones. I'm not exactly sure how sticking one on some kind of mount immediately above several tons of steel may change its characteristics.

 

I tend to think a ground plane type antenna may work better in such a case. Certainly the cut down 2 metre whip on a mag mount that we use has proved itself very well.

 

Generally speaking, I would have though that where you need such an antenna deployed, you are not regularly going to be going under low enough bridges to cause problems. OK we have to lay our antenna flat as soon as we (say) turn off the Thames at Brentford, (very low bridge at certain tide conditions), but the river itself has ample headroom, of course.

[Alan de Enfield]

I got a 1/4 wave mag-mount taxi aerial off ebay (about £20 I think) - the seller changed the plug over to a PL259 (free of charge), cut the whip to length and it works perfectly.

 

Its 'short' and doesn't cause any problems, it reaches from one lock to the other on the Trent, & it lifts off and stows easily for canal use.

[by'eck]

I may be missing something, but doesn't OP already have a dedicated antenna that can be used in exactly that way, and is not relying on the boat for a ground plane ?

 

I guess my only concern, (and I'll admit that I am more out of my comfort zone here!), is that I would guess antennas like both he and you have pictured usually get strapped to the top of GRP boats, not massive steel ones. I'm not exactly sure how sticking one on some kind of mount immediately above several tons of steel may change its characteristics.

 

I tend to think a ground plane type antenna may work better in such a case. Certainly the cut down 2 metre whip on a mag mount that we use has proved itself very well.

 

Generally speaking, I would have though that where you need such an antenna deployed, you are not regularly going to be going under low enough bridges to cause problems. OK we have to lay our antenna flat as soon as we (say) turn off the Thames at Brentford, (very low bridge at certain tide conditions), but the river itself has ample headroom, of course.

 

Fair comment but the base loaded whip shown provides its own virtual ground plane (kicking off surface for the signal) so the surface its was mounted to would have little impact on performance. If you are going to spend money on a permanent mount VHF set with 25 watt output, it surely makes sense to use its full capabilities, hence comments & suggestions.

[Nickhlx]

Yes - The Metz is what I was describing - half wave element, end fed, and with a matching coil to transform the high impedance at the end of a 1/2 wave to the 50 ohms of the transceiver.

 

The issue I have with using PL-259 / SO-239 outside is that they are not waterproof, so at the very least they will need wrapping up in self-amalgamating tape to seal out the rain / corrosion, and then overwrapping with black insulating tape, as it ( S.A. tape) is not UV stable.

 

Don't be over-enthused by the dB gain figures... the Metz is a 0 dBd ( they "conveniently omit the second, all-important "d") as is a 1/4 wave whip, whereas a 5/8 whip is 3 dBd gain.

 

Where the Metz and other 1/2 wave aerials do have a benefit is that their main lobe is at 0 degrees ( horizontal ) but only in free space which in practice is well above the ground or a boat roof. The 5/8 whip which (conveniently) does need to be on the boat roof, has a 3dBd gain ( i.e. double the gain) and its lobe is at about 30 degrees, which helps if you are in a low down location ( canal in a valley) as the Metz will fire all its signal horizontally

 

We are nit-picking here with the differences as the practical considerations will have far more impact on the choice and to how well it works. However my first choice would be to fit a 5/8 whip with a tilt over mount for best performance and practicality, as the electrical connections would then be on the inside of the roof, it would be easy to lay flat for bridges ( if you needed to) and would avoid the need to work out a way to bring the coax inside with a waterproof gland, which is why most commercial and amateur users choose it.

 

Nick

Edited January 3, 2012 by Nickhlx

[by'eck]

If you can get hold of the shiny red self amalgamating tape, it is UV resistant, as I have proven in the Caribbean.

 

I note that this black variety is also claimed to be UV resistant.

 

BTW Nickhix, dBi is the usual reference when discussing antenna performance, comparing to an isotropic radiator. dBd is its performance relative to a dipole (2.15dbi) & usually only used by radio hams who are familiar with such. Sorry to be pedantic but you brought it up

Edited January 3, 2012 by richardhula

[Giggetty]

I have just bought a new radio which comes with a 'flat surface' mounted antenna. I asked about a magentic base...as I didn't want to bolt it onto the boat all the time...but they don't do one.

There are several suppliers of CB mag bases on Ebay....Just the bases...wired complete with RG58 cable and PL259 plugs.( I KNOW it will damage the radio to use a CB mast....don't worry)

My question is...would it be alright to use one of these mag bases...adapted to screw my new antenna into the base...as long as I don't lengthen the operating length of the aerial?

 

Thanks.

 

Bob

Assuming that your radio is a marine VHF transmitter, just buy from Maplins a magnetic-mount 2m band amateur radio whip antenna. Cut it down with side cutters progressively until SWR<1.5 on channel 16. I think I removed 1 1/4" this way.

Edited January 3, 2012 by Giggetty

[Nickhlx]

If you can get hold of the shiny red self amalgamating tape, it is UV resistant, as I have proven in the Caribbean.

 

I note that this black variety is also claimed to be UV resistant.

 

BTW Nickhix, dBi is the usual reference when discussing antenna performance, comparing to an isotropic radiator. dBd is its performance relative to a dipole (2.15dbi) & usually only used by radio hams who are familiar with such. Sorry to be pedantic but you brought it up

 

 

 

Interesting about the red tape - I will see if I can find out more about the suggested black variety..

 

I think the dBi term is pretty meaningless in practice - Sure, it's a baseline from which to measure, but that is about all... It is avoided by those wanting a measurement of worth of the performance of an aerial ... Naturally the manufacturers will use it as it will result in a bigger number for the gain of their aerial, but for those just "dB", I think it is almost tantamount to deception , although, I suppose no worse than being economical with the truth. However, for those not familiar with the differences, and not knowing to question the comparison ....... !!

 

As we know, if a figure is claimed as " x dB gain", it clearly is very unlikely to be dBd and is like saying it has "3 times the gain"... compared to what, and is the gain in a wanted or useful direction ? Proper (Professional?) antenna manufacturers usually advertise their kit properly though... I don't think its being pedantic, it's part of the necessary information people need if they want to be able to compare products properly...

 

Nick

[Bobbybass]

Assuming that your radio is a marine VHF transmitter, just buy from Maplins a magnetic-mount 2m band amateur radio whip antenna. Cut it down with side cutters progressively until SWR<1.5 on channel 16. I think I removed 1 1/4" this way.

Thank you...but I HAVE an antenna with the new radio.I was just asking if I could affix it to a CB mag base. If I buy another antenna and cut it down...extra money..I am then into another £30+ for a SWR meter...no ?

 

Its all getting...out of control...no ?

 

Bob

[Alan de Enfield]

I am then into another £30+ for a SWR meter...no ?

 

No - if you trim the wire whip to the correct length you can get away without SWR'ing it.

 

If you have a base coil (or coil in the midle of the aerial) then you cannot measure the length. A straight steel whip is easy to measure and trim to length.

 

Whip length....frequency MhZ

450mm....... 156 mhz

440mm....... 158 mhz

433mm....... 160 mhz

 

Channel 74 is 156.725, Channel 16 is 156.8 whilst Channel 50 is 158.45

 

Wherever you trim it to it will be 'wrong' for any other frequency so just take a middle-of-the-road length or trim to the frequency most widely used in your main area of crusing.

[grandbanks]

I have just bought a new radio which comes with a 'flat surface' mounted antenna. I asked about a magentic base...as I didn't want to bolt it onto the boat all the time...but they don't do one.

There are several suppliers of CB mag bases on Ebay....Just the bases...wired complete with RG58 cable and PL259 plugs.( I KNOW it will damage the radio to use a CB mast....don't worry)

My question is...would it be alright to use one of these mag bases...adapted to screw my new antenna into the base...as long as I don't lengthen the operating length of the aerial?

 

Thanks.

 

Bob

 

Yes, Bob, you can use a any mag mount. Get one that has a 16mm threaded male connector that will screw into the SO239 socket in the base of your antenna. Your antenna may have a barrel connector screwed into the SO239 socket in the base of the antenna, in which case just unscrew it. The mag mount should have a cable terminating in a PL259 which screws into the SO239 socket at the back of your radio. The length of the cable from antenna to radio is not critical and you don't need to worry about ground planes. You don't need an SWR meter.

[grandbanks]

 

Don't be over-enthused by the dB gain figures... the Metz is a 0 dBd ( they "conveniently omit the second, all-important "d") as is a 1/4 wave whip, whereas a 5/8 whip is 3 dBd gain.

 

 

Nick

 

 

Hi Nick,

No, we don't 'conveniently omit the second, all-important "d". We use dBi as the reference on our Metzeurope site and on our Salty John retail site. dBi, gain over isotropic, is the standard reference in the marine industry. dBd, gain over dipole, is used more often by CB'ers.

If you can find, on a Metz or SJ ad, a reference to gain that does not include the all important "i" please let me know and I'll have it changed. Of course, some retailers who stock the Metz may use their own text and we have no control over that.

As you know, a high gain is not necessarily better than a low gain. I'd prefer to describe it as broad beam (low gain) or narrow beam (high gain) to make the concept easier to grasp for the general public. On a boat, especially if the antenna is located high up, you don't want a high gain, narrow, radiation pattern because as the boat rolls the signal will be pointing at the sea or the sky and not at the horizon. A broad, lower gain, pattern will always have some portion of the radiation pattern pointing at the horizon.

I have only felt the need to clarify this because you singled out Metz for criticism, somewhat unjustly in my view. The Metz Manta is a superb VHF antenna, the only one offering a lifetime warranty, and used by US and UK search and rescue services.

[Nickhlx]

Hi Nick,

No, we don't 'conveniently omit the second, all-important "d". We use dBi as the reference on our Metzeurope site and on our Salty John retail site. dBi, gain over isotropic, is the standard reference in the marine industry. dBd, gain over dipole, is used more often by CB'ers.

If you can find, on a Metz or SJ ad, a reference to gain that does not include the all important "i" please let me know and I'll have it changed. Of course, some retailers who stock the Metz may use their own text and we have no control over that.

As you know, a high gain is not necessarily better than a low gain. I'd prefer to describe it as broad beam (low gain) or narrow beam (high gain) to make the concept easier to grasp for the general public. On a boat, especially if the antenna is located high up, you don't want a high gain, narrow, radiation pattern because as the boat rolls the signal will be pointing at the sea or the sky and not at the horizon. A broad, lower gain, pattern will always have some portion of the radiation pattern pointing at the horizon.

I have only felt the need to clarify this because you singled out Metz for criticism, somewhat unjustly in my view. The Metz Manta is a superb VHF antenna, the only one offering a lifetime warranty, and used by US and UK search and rescue services.

 

Hi,

 

 

Thanks for the info...

 

The ad I saw where the "i" was missing was here http://www.metzcommunication.com/images/antennasheetforweb.jpg when comparing with a 54" 3dB aerial.. which naturally caught my attention.. Not sure if you have any influence over that advert... I can't comment on the use of dBd by CBers as don't really follow that branch, more the professional and amateur products...

 

However, I would like clarification on how the PL-259 / SO-239 antenna connector can be reliably considered for use as an external connecting system as the ones I have seen are not waterproof or sealed, and I would never use in an external let alone a marine environment...Over time, salty water must creep into both the threads, and between the centre contacts, and very likely into the co-ax braiding. This would be my only criticism of the Metz . It could be overcome with either a change in connector type, suitable external self-amalgamating tape, or perhaps terminating in a length of co-ax that could be brought into a suitable sealed connection box. Perhaps this could be incorporated into the base of the antenna ? However, perhaps the type of PL-259 that the Metz uses/recommends is a sealed item ?

 

My suggestion for the 5/8 was exactly for the reason you confirm, with a bit of vertical component in the beam pattern as well and as you say it is probably easier for the public to grasp.

 

I realise the above points are possibly going to be taken by most people as un-necessarily pedantic, but the work I do demands a zero failure rate and the best practices available, and I would agree that it may be considered financially unviable for most commercial products where a reasoned balance needs to be struck. I apologise if my comments were taken to cast doubts on the product, but were based on past experience of the PL-259 in an external environment - I would not be put off buying one myself, although may make the above mods depending on the application...

 

With regards,

 

Nick