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 Suggest a mobile antenna please.... 
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Post Suggest a mobile antenna please....
Due to a proposed house move I've had to take down all antennas and consequently after a month of inactivity(apart from a pathetic fishing pole 20 metre vertical loop) I've gone stir crazy to the extent of driving down the lane away from the houses and working 10 metres on my old CB radio junk......it feels so dirty :eye:

Anyway having initially zapped my hand from the chrome button in the back of the mic.......stray RF :oops: it got a bit better today when I took the car radio antenna off the back of my Zafira and installed the 3/8 mount from the CB mag mount through the roof hole instead having sanded the roof back to bare metal on the inside and added a large bright zinc washer for strength,better, to the point that I actually worked a few French and Portugese stations.

Flushed with this success I'm thinking what if I run an HF radio mobile....well parked up mobile with the engine off that is as the Zafira seems to be an excellent source of QRN......what if I screw longer antennas onto the 3/8 mount and go for it on the lower bands eg. 17 and 20 metres?

My thoughts are at 16 ft high a 1/4 wave vertical is possible when parked up as long as the wind doesn't blow it over :cheese: and all higher bands will be successively shorter so less problematic,given that my projects are always rule of thumb/clumsy rather than technical I'm thinking a 1/4 wave will be more straight forward to construct rather than all that working out and measuring with non existant test equipment that would be required with a slightly shorter badly mutilated/modified ex CB antenna.

What doe's the team think am I a wimp or a gung ho pipe wielding maniac......or should I take the house off the market and put my Moxon back up......let's face it I'm not going to sit in my mobile at 1:30 in the morning talking to a friend in the States......I miss my antennas! :cry:

Any suggestions on base/mid loading would be great if they were backed up with suggested winding info rather than one versus the other arguments also bear in mind that Dorset lacks anything resembling a Ham emporium so this will be a Screwfix/BQ lash up.

Looking for an house with a bigger back yard BTW,cheers in anticipation, Dave.


Tue Aug 03, 2010 11:46 pm
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 Suggest a mobile antenna please.... 
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Post Re: Suggest a mobile antenna please....
homebrew multibander using readily available stuff.. an old pro am whip stripped down, some waste pipe, wooden skewers,wire and a crock clip and a bit of glue and a few electrical connectors..

tunes 40-2m , cost around Ā£20 including the antenna to strip down.

billy


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Wed Aug 04, 2010 3:01 pm
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 Suggest a mobile antenna please.... 
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Mags are not the best solution to mounting an antenna although i do use one on the roof of my van but as you see in the picture its connected to a body mount drilled to the roof for a physical earth connectuion . this was because the mag offered much more strength than the small body mount and my roof is literally an 18 inch strip of metal , the rest are sunroofs.

my main mount is a body mount on the rear quarter panel.

i know daves already made the jump and drilled a hole :lol: something lots of people are scared of for some reason.

do some bonding of body panels now you have got a decent mount and your half way to a decent system.. it really is time well spent and if done properly should never be seen or at least questioned about by a propspective future buyer .


billy


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Voice is for CBers, amateur radio operators, the average citizen, and the military. In other words, voice is for everyone with a mouth. CW is for those who choose this newer mode of communication. Newer? Why yes. Voice has been around for a million years.
Wed Aug 04, 2010 7:20 pm
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 Suggest a mobile antenna please.... 
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Post Re: Suggest a mobile antenna please....
Thanks for the replies gents although I haven't been quite as brave as Billy think s as I only removed the factory fitted bee sting antenna and replaced it with the antenna mount that I removed from an old CB magnetic mount.....I appreciate how people feel about mag mounts and if I was as brave as Billy there would be a hole in the centre of the roof rather than the factory position just above the hatchback.

If I could ask Billy, the coil on that businesslike antenna of yours looks to be wound around some sort of skeleton frame I'm guessing to improve inductance but without an antenna analyser here where would I start with constructing a coil with regard to achieving an impedance match to the 50 coax and making sure that the antenna is resonant on the chosen band?

I'm revealing my ignorance here and you can see why the 1/4 wave idea is attractive albeit on 20 metres the height will put a tiny bit of mechanical strain on the 3/8 mount as well as look a bit odd :eye:

I could do with more info on your antenna Billy as I'm happy building stuff, just can't help thinking that looking like it works and actually working are not always the same thing.....all noted regarding the electrical bonding of the various car panels to increase the groundplane side of the dipole.

Cheers,Dave.


Wed Aug 04, 2010 8:49 pm
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 Suggest a mobile antenna please.... 
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Hi Dave, well you swapping the antenna mounts is still a move in the right direction as now you have a mount thats bonded to your bodywork..

RE the antenna .. although i did have an analyser i sold it ( one of my more stupid moves) so this was made without one.

it was made "blind" as in i knew an 8 foot whip wouldn't require that much loading on the higher bands , especially base loading. i knew center loading would require a bit more for the same given band but wasn't sure how much..

SO to be honest it was just a damn good guess.. My idea was to simply fill the former with turns and see what i got and work from there..

The coil former is made from plastic waste pipe with wooden barbecue skewers as standoffs so the wire is sat off the former to allow a crock clip to be easily clipped on.

The pic you see with the single coil will tune 40 up as a portable antenna which is what it was originally made as ..

i decided to try on my van but was a little out to get 40m so simply added a small loading coil at the base which is clipped in line when tuning 40m on the main coil and unclipped for all other bands whilst being used mobile.. for /p the bottom small coil is not needed.

id be more than happy to send you detailed build instructions .. i would put it on here but i could draw it and explain it better on a piece of paper..

unless of course you would prefer it over here in which case ill take some piccies tomoz and post it , im easy which ever way..

billy

pic is of completed antenna with base loading coil added for /m use.. this could be ommited simply by extra turns on the main coil but couldn't bring myself to take it apart..


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Wed Aug 04, 2010 10:06 pm
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 Suggest a mobile antenna please.... 
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Got a chance to play with the CB antenna today and as there was a contest running there was a lot of refugees on 17 so that was the target band to bend this antenna towards.

I'm currently limited to an SWR meter so that's how I tweaked the antenna and the tweaking such as it was consisted of making a four leaf clover capacitance hat on the end of the whip.

I appreciate this was probably not the right thing to do but as far as the radio was concerned it was looking at an SWR of 1.55 :1 in some sort of combination of coax,antenna and car body to ground coupling.

The antenna is slightly over 2 metres long with a loose helically wound coil of 16 turns over a foot of fibreglass a mid section of 18" of stainless whip another loading coil identical to the first and then 18" or so of whip to which I've attached the capacity hat which is a four leaf clover shape about 2 feet across.......it worked but would it be a fair guess that not much of the signal was actually leaving the antenna.......talked to AB5LZ on it, albeit only for about three minutes! :eye:

Cheers ,Dave.


Sat Aug 07, 2010 7:06 pm
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 Suggest a mobile antenna please.... 
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My new mobile antenna is for fixed mobile, its too big to drive with.

The bottom section is probably a G Whip section borrowed from Norman G3ZWR, with a 3/8 thread at the bottom and a 3/8 thread socket at the top, this thicker section is about a metre long.

Screwed into this is the top whip is called a CB tank whip, and is a quarter wave on 27 Mhz.

The whole lot is resonant as a quarter wave on 19 Mhz. It tunes up on 18 Mhz and 21 Mhz as well as 24 Mhz !. The SWR on 18 Mhz may be as high as 2 to 1 but the auto atu copes with that.


Picture of the Antenna.
Image

The antenna has no coils and works DX on 18 Mhz, best location is parked up near the sea.


Tue Aug 10, 2010 6:48 pm
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That's the sort of thing I've arrived at now as the G Whip bottom section would normally be helically wound making the two helically wound sections on my old CB antenna similar in theory?

Either way ironically I've got a 9 ft 1/4 wave whip (two part) on order right now as I was thinking of introducing an air spaced coil....actually the air spaced coil out of my broken MFJ ATU at the centre join section.

Not too sure how this will happen but obviously with the section inside this being RF invisible/insulated I imagine it will be the best I can bodge in fibreglass or plastic with a flying lead arrangement to bring more or less coil in circuit with the antenna.

I like the idea of the antenna being resonant on each band rather than using an ATU in the car but that's due more to wishful thinking as at the moment I've only managed to get the antenna performing on 17 metres.

I was hoping Steve would cast a technical eye over the thread and indicate what might be happening with the capacitance hat arrangement bringing the CB antenna down to 17 metres although as the SWR curve is quite wide giving about 1 MHz of bandwidth centred roughly on 18 MHz before it goes over 2:1.....I appreciate that this probably indicates a lossy antenna.

At the moment I'm finding it difficult mentally to agree efficiency with resonance and providing a 50 ohm coax match and the more I read the more it seems that a lot of the wonder mobile antenna home brew designs don't address all criteria.

I do have limited access to an MFJ 259 so there will be a certain amount of indicating what's going on.....hopefully!

I'm going to have to try the sea front location too as we were down by Lymington Sunday taking the bikes out for a ride along the sea front and frequently get down to Weymouth which is only about 25 miles south of here,cheers for the reply and pic as like the ones Billy posted they can put things into perspective rather than simply an idea floating around.

Dave.


Wed Aug 11, 2010 8:13 am
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 Suggest a mobile antenna please.... 
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G7VQE wrote:
I was hoping Steve would cast a technical eye over the thread and indicate what might be happening with the capacitance hat arrangement bringing the CB antenna down to 17 metres although as the SWR curve is quite wide giving about 1 MHz of bandwidth centred roughly on 18 MHz before it goes over 2:1.....I appreciate that this probably indicates a lossy antenna.


Dave,

I was keeping quiet because the thread seemed to be going along nicely on its own :)

I probably don't have sufficient data to fully model your antenna - helical loading is particularly difficult - but bringing it to resonance with a capacity hat must be a good thing.

Getting electrically short antennas to be efficient is all about increasing the radiation resistance (Rrad) and reducing losses. Here are some general pointers:

* The longer the whip, the higher Rrad is
* Top loading (particularly a capacity hat) produces the highest Rrad possible
* Mid-loading is next best
* Bottom loading produces the lowest Rrad

* Air-cored loading coils wound with large diameter wire will have lowest losses
* Don't forget losses in the grounding - they are also part of the antenna system

To put things in perspective, here are the Rrad values for a 6ft whip on various bands:
160m 0.1 ohms, 80m 0.24 ohms, 40m 0.82 ohms, 20m 3.2 ohms, 10m 14.5 ohms

Any ground resistance or loading coil resistance which is comparable to those values will have an adverse effect on efficiency.

Finally: the more efficient the antenna, the higher the VSWR will be (before any matching) and the narrower the bandwidth.

Hope that helps some.

Steve G3TXQ


Wed Aug 11, 2010 8:39 am
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 Suggest a mobile antenna please.... 
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Dave and Steve,

Yes agree with Steve about a capacity hat. I have no windings or coils as part of the antenna.

If you do not want coils you can purchase 3/8 24 steel rods at 3 foot and 6 foot. (Google) - These seem to be threaded along their total length, which should work, but might be heavy ?. There are also Nylon Threaded Rods , 3/8"-24, 12" Length (Pack of 1) from garden suppliers if you really want to wind coils, (as Steve says the higher up the coil is the better - there may be mechanical constraints with some arrangements).

If anyone can source steel or ali sections 3ft or 6ft with a 3/8 (24) male at one end and 3/8 (24) female at the other to extend the tank whip I would be grateful for the details, as this would allow very cheap full size HF 1/4 wave verticals for 14 Mhz (capacity hat) to 21 Mhz including 18 Mhz. Am I right in thinking you could have a resonant 1/4 wave on 21 Mhz and simply add a modest capacity hat to resonate it on 18 Mhz.

Steve
G4AZB


Wed Aug 11, 2010 12:23 pm
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 Suggest a mobile antenna please.... 
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The total length of my whip is about 12 feet at present and the antenna analyser says it is resonant on about 19 Mhz. It survives the gales when on top of the car at the lighthouse.

1/4 Wave Mobile antennas for HF Bands (phone sections) would be about (start longer and trim)

28.500 Mhz 8ft. 2 - 17/32in. Tank whip only.
24.950 Mhz 9ft. 4 - 9/16in. Tank whip only. (may also match as a 3/8 wave at longer lengths)
21.250 Mhz 11ft. 0 - 5/32in. Tank whip + 2 foot approx (a 5/8 wave on 6 metres with matching coil at base)
19.000 Mhz 12. 0 Tank whip + 3 foot approx G4AZB/ M 's current "in between" bands antenna
18.150 Mhz 12ft. 10 - 23/32in Tank whip + 4 foot approx
14.250 Mhz 16ft. 5 - 1/16in. Tank whip + 6 foot approx (w Capacity hat) or 7 foot 5". (w/o hat)

Steve


Wed Aug 11, 2010 1:06 pm
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Post Re: Suggest a mobile antenna please....
Thank you for the replies gents and I have to say it's all gone a bit astray tonight what with fitting a more solid 3/8 body mount fed underneath the roof via a PL259/SO239 connection and running a new coax......why ruin one of the parameters when you can throw the lot out the window?

Luckily I'm off work tomorrow so hopefully I'll get time to unravel all the good work of this evening and find out why the SWR has gone west.....at the moment it's not producing the same result twice so a post mortem is required before damage occurs.

The main reason I'm dwelling on the capacitance hat in particular is probably from the AC theory learned back in the late 70s during my electrical apprenticeship....for some reason this unlike most subjects has lodged in my head but from those days we added capacitance to compensate for heavy inductive loads in an effort to bring the circuit back to resonance (unity) and basically get back some free electric as far as the metering equipment was concerned.

In the case of my old CB antenna I had no idea what load it was presenting on 17 metres but added the loops one at a time at the antenna tip until the antenna SWR got to 1.55:1 which was close enough for a bodger like myself, however I must I was always under the impression that an antenna that was too short presented a capacitive load which was why inductance was introduced to bring the antenna back to resonance? :eye:

Either way I'm not giving up yet but any thoughts of getting a 59 report from some distant station are fading rapidly after tonights fiasco.....out of interest the antenna wouldn't even give a sensible reading on 10 metres minus the capacitance hat so it's looking coax and mount related.

I've also got the two part 1/4 wave CB whip now so may even start work on the next pile of s.......antenna but obviously have to get back to a level playing field first.

Cheers and thanks for the pointers Steves,Dave.


Wed Aug 11, 2010 8:44 pm
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 Suggest a mobile antenna please.... 
Silent Key

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G7VQE wrote:
In the case of my old CB antenna I had no idea what load it was presenting on 17 metres but added the loops one at a time at the antenna tip until the antenna SWR got to 1.55:1 which was close enough for a bodger like myself, however I must I was always under the impression that an antenna that was too short presented a capacitive load which was why inductance was introduced to bring the antenna back to resonance? :eye:


Yep, it's confusing isn't it. Think of it this way:

When you create capacity at the top of the vertical by adding a "hat", current in the tip of the whip has somewhere to flow; so the length of the whip acts like an inductor. With enough capacitance at the top you can tune the inductance of the whip to resonance - you've created a series tuned circuit.

If you have no hat, there's nowhere for the current to flow at the top, and the whip acts like a capacitor. You bring it to resonance by adding a base loading inductor in series with it at the base.

It's not quite that simple, but it'll do ;)

Steve G3TXQ


Wed Aug 11, 2010 8:58 pm
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 Suggest a mobile antenna please.... 
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thats why i like my center loaded.. simply tap for lowest vswr .. i have no analyser again yet so this is good enough for me for now . when i get an analyser again i will resonate and match to coax on the bands needed with an 8 foot whip will be 40,30,20 and maybe 17 depending on whip section length..

i had an 8 foot tank whip but found it far too heavy for my liking even as a base section .

heres my old first attemp at a mobile antenna .. i used to get some comments on the size of the coil and the use of the magmount frame for earthing etc from the magmount boys.. :lol:

the rear mag was taken off and connected to a boot mount modded to purely use as an earth point which in turn was earthed to the boot then the usual bonding from there . a steam train couldn't of parted the panels on my old motor because of earth straps..

i could of kept a canary inside that coil :lol:

billy


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Wed Aug 11, 2010 10:24 pm
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 Suggest a mobile antenna please.... 
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Post Re: Suggest a mobile antenna please....
Back to the drawing board or more likely back in the shed for the modified CB antenna.

It's been a strange one this as while I got a chance to check both capacitance hats on the antenna in situ with an MFJ 259 and they look good the reality is that when used with the radio one second the SWR is normal the next time the mic is operated then the SWR has gone to pot and the antenna isn't resonant on any band so not simply a make and break connection between the hat and the whip causing it to swap between 10 and 20/17.

The behaviour is almost like that of some sort of static discharge with the next transmission seeing the SWR back to normal albeit subsequent times the SWR will go out the window again.......just when I thought I had reinvented the wheel too. :cheese:

I've got to work the next couple of days so can't play antennas again until Sunday and after messing around all day (wife not very happy) with this conundrum and getting nowhere apart from some European contacts on 17 this afternoon using 50 watts.......the problem seemed related to increased RF power at that point.

It's probably best to give the single centre loading coil or straight 1/4 wave vertical antenna a try now before I give up on mobile HF forever and put my mast and Moxon back up and actually work what I can hear. :oops:

I have to admit I'd like to get a handle on what's causing the problem with the antenna as it seemed to be going so well at first.

Cheers in anticipation,Dave.


Thu Aug 12, 2010 10:01 pm
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