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SWR'ing a K40

Dal

Dal

New Member
Jul 12, 2016
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I am a novice so please don't tear me a new one if this is a daft question...

I have a President Grant II running straight out to a K40 antenna. This rig is mounted in a 1976 Mini Clubman Estate and I have the K40 roof mounted at the front (reusing the hole formerly occupied by the car radio's antenna). No amp or any of that shizzle.

I use the SWR meter in the radio for my readings and I have not modified the coax on the K40.

With the radio in 'stock' mode, the SWR reading on UKFM channels was a little on the hot side, but workable (Ch1 reading a little over 1:1, Ch20 at 1.5 and Ch40 just under 2). My antenna is as short as it will go before cropping the whip.

Then I switch the radio into Export mode and I test UKFM again to find Ch1 = 1.5, Ch20 = 2 and Ch40 = 3. Giving me only half the band but with a bit more power.

In export mode, I can freeband, so I hunted around to find the upper and lower bound frequencies the K40 will work with (where the SWR hits 2). The frequency range appears to be 26.735MHz - 27.74125MHz.

So this setup is only giving me a little over 1MHz of usable frequency range. Seems odd as the K40 is supposed to be rated 26-30MHz and should give fairly consistent SWR (if the marketing is to be believed?).

I'm thinking of adding an antenna tuner, but not sure if I'm just 'band-aiding' a more fundamental problem. Any help / advice would be appreciated!
 

Dal said:
Seems odd as the K40 is supposed to be rated 26-30MHz and should give fairly consistent SWR (if the marketing is to be believed?).
Click to expand...


That range is the useable range that the antenna can be tuned too but not the entire workable range with just one setting. IOW yes it will cover 26-30 MHz but only in multiple segments ie. pull it out all the way and it may cover 26-27.4 MHz. Push it in all the way and it may cover 28.5-30.0 MHz. You will NEVER cover the entire range of 26-30 MHz with a low SWR with that (or pretty much any) antenna.
 
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Thanks for the clarification - I accept that antennas are to be tuned for their range and that an antenna's length will really only have a sweet spot of one frequency (according to maths).

I guess my issue is that I have the antenna at its shortest possible length and still my highest workable frequency is 27.74125MHz which is some way shy of the rated 30MHz. Add to this is my concern that the SWR changed drastically when using the more powerful export mode. Given this antenna is rated at 500W, looking at the difference after a small change in power output, I struggle to see how I would even be able to run 50W through it. I guess I feel the working range of this thing is smaller than I was expecting.

Hoping someone with a K40 can confirm or refute my findings here so I have something to go on.
 
You'll need to cut the antenna to get to 30mhz area. Or near there. Just like Captain Kilowatt stated, antennas like yours will only be resonant or perfectly or as near as the antenna can realistically get to its best readings will be to cut the antenna for one specific frequency. Then go from there. Most every stock whip I have ever bought has needed to be cut for a proper tune and R and X readings as well as VSWR, on an antenna analyzer. And this was for a set frequency like 27.405. You will then have a little bandwidth to play with either side of that set frequency point above. And it all depends on the antenna as to how much bandwidth you'll get. How it's built and such.
But like said, don't go thinking you'll have a good VSWR from 26-30 MHz because of what is written on the box. All that is telling you is the antenna can be tuned to work from 26-30 MHz. They don't specificy that you have to actually tune the antenna to a set frequency. There is more to antennas then just VSWR readings!! And the K40 is a POS antenna IMO as well. Inefficient and made like junk. I'm sorry if I am being harsh. I have one sitting here in my garage that I just keep for spare pieces for other things. You want a better antenna look at the Sirio performer 5000 or predator 10K. Or even a 108" whip or a 102" whip and a good SS Spring. I had a K40 and it just didn't cut it for me. Replaced with a Sirio 5000 performer and it made a large difference.
But like I said from the get go, even the Sirio antennas have to be cut and set for a specific frequency and then you will have bandwidth on either side of the resonant tuned frequency. How much bandwidth is hard to say as every install is different. But I would think 1 MHz of bandwidth should be attainable. Or maybe closer to 2mhz.
You need an antenna analyzer, and know how to use it, to really tune and antenna properly. This is JMO. And the above are just from my own experiences with the K40 and other antennas as well. All the rest of the antennas I mentioned will work better then the K40 IMO. Height is might!! And the closer to a 1/4 wave antenna you can get, the better. Physical length, not what the box says either. The closer to 9ft. Approx. 8ft 6in or so, depends on where you plan to use the antenna the most to talk frequency wise.
Sorry for the text wall, but like I said there is a lot more to tuning an antenna properly then just a VSWR (swr) reading.
Do some research on what VSWR means and how other things need to be taken into account when tuning an antenna like your K40 and most any other antenna for that matter. Capt. Kilowatt knows his stuff inside and out, so listen to him closely. He speaks the gospel about this stuff LOL!! Preach on CK!!:D:D;)
 
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Thanks for the good advice. I just wanted to be sure I was getting what is expected from this thing which it sounds like I am.

I have been reading and studying this page this morning:
http://www.hamuniverse.com/wc7iswr.html

Chimes in well with what I'm seeing and good to understand the difference between 'antenna' SWR and 'antenna-system' SWR as well as making sense of the ratios and how to calculate the reflected and re-reflected power.

I'll get a simple tuner for this and may well look at the antennas you mention. This is going on a car so don't really want a 9' whip on it - therefore I must compromise.

Thanks again!
 
The Sirio performer 5000 is taller then the K40. It's almost 7ft, but is a nice antenna. I ran a K40. My biggest issue area right off the bat was the 2 piece connector supplied with them. The SMA to PL259 or something along those lines mine was. They go bad very easy and would be the first thing I would change out. As far as the rest of the things. You really should need a tuner to talk on the 40 channels of the CB band. To each his own. Just be safe and have fun I suppose. Good day. And like I said, there's more to it then just VSWR. You need to look into reactance and impedance matching as well. Good day.
 
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I wouldn't rely on the built-in metering of that radio or any other one. Get a good, brand-name SWR meter, treat it gently, and it'll last for a long time. I don't know what's available where you are so I can't recommend anything. Use the built-in meter just as a relative indication, not for any quantitative data.
 
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If you've mounted it on the roof through a hole you're going to end up cutting the whip. The K40 usable range applies when you've a piss poor RF ground but have a good one like yours and you'll find that:

1) The whip is too long for efficient installations and you'll need to cut it. Manufacturers make whip lengths that will work with the worst installation possible which means they tend to be a little long. I just bought a Sirio Performer 5000 this last weekend and mine is mounted properly like yours but in the middle of the roof. I ended up cutting off over 3 inches and its still about 400-600kHz too low for where I want it. Out of the box with the whip fully lowered it was tuned for 26.6MHz, well below even the EU CEPT CB band. With 3 inches cut off it its now gone up to 27.400MHz. Ideally it wants another two inches chopping off to raise it up to 28MHz so I can use it for both the 11m UK CB band and 10m amateur radio band SSB portion. I have an antenna analyser so I've no qualms about getting the dremel out and lopping off what would seem like a crazy amount to someone without an analyser.

2) It won't be as broadbanded as the manufacturer claims because its more efficient and much of that claimed bandwidth is due to losses in the antenna system and usually RF ground losses because people use magmounts, mount them on roof racks and other such rubbish.

As for the 30MHz, unless you have an amateur radio license you do not want to be transmitting higher than 27.997MHz. 28.000MHz to 29.600MHz is the amateur radio 10m band. Go on there without a license and they will report you.

As long as the SWR is below 2:1 on the frequencies you want to use it'll be fine. Don't be obsessed about trying to get it down to 1:1. If you did achieve 1:1 it would actually be an indication of a lossy installation.
 
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Thanks - I am taking my foundation exam in a couple of weeks. Then going for the intermediate next month, but appreciate the warning ;)

All good advice and based on the above, I have ordered a Sirio Performer 5000 and a Zetagi HP-1000 which I'll stick inline.

Interesting idea about covering the 11m and 10m with the same antenna. Once I've invested in a proper analyser (or found someone local with one!), I'll probably go down the same route.

Love this forum!
 
the older k 40 base part the cap was wrapped and soldered,, the newer ones is just a hook and soldered alot of wattage would heat it up and solder melt and cap would come off,,,been there done that,,,,,
 
Sirio Performer 5000 and Zetagi HP-1000 installed and it is a world of difference. The antenna tuner - while probably overkill for CB - appeals to my pedantry. I can tune right across the EU and UK bands with good SWR to the radio now.

In real terms, I'm now getting out with better modulation reports and better range.
 
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Excellent news. Your original post indicated you'd done a pretty good job of installing the antenna you had.

Last thing to do is a bit of bonding and on your Mini it would be across the hinges on the bonnet and maybe run another ground strap made from flattened coax braid from the engine to the car body which should help with any engine noise you have.

A bit of reading about it all:

http://k0bg.com/bonding.html

I make all mine out of flattened RG213 braid but you can buy pre-made ones at B&Q and Ebay. You want the ones which are flat braid rather than wire.
 
Yup, the Sirio performer 5k is definitely a step up from the K40. Mine was a POS!! It leaked at the joint and the 2 piece connector was total junk as well as not tuning worth a crap even with a large ground plane under it. Many recommended the Sirio performer 5k to me and it has been one good antenna for me and about 4-5 others locally as well. They do a good job. Installing them properly like M0GVZ and others have stated is the key to success! Glad you got it figured out and are on the air. (y)(y)
 

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