Blues with Diamond CP-6 Antenna

g0mry

g0mry

Member
Hi,
ive got a Diamond CP6 and have got a problem with the swr
across all bands its around 12.1
now antennas is not my thing so any help would be great.

Ive checked and double checked the measurements tried different positions in the garden but to no avail
ive tried to seperate leads around 40-70 ft long coax although they are 15years old! but on the meter check as fine

the only band I get a 1.1.1 match is 6mt around 51mhz

ive run out of ideas so any of you can help a old frustrated ham please do

the swr has always been high but have used a tuner to combat this, but it's now got to the stage where a tuner will not give me a low swr so cannot use my flex3000
help

Mike
G0MRY
 
Hiya Mike

Sorry to hear about your sick CP-6 antenna. The Diamond CP-6 is normally pretty good, and the only problem seen with some of them is where water has found its way into the tuned radials.

I'm not sure if you are aware of it, but the five loaded radials (six meters uses an unloaded telescopic radial), have two small holes in the covers of the loading coils. These holes are to allow any water to drain out of the loading coil cover tubes, and according to the instruction sheet they must point downwards. Also, these holes allow the loading coils to 'breath'.

If you happened to mount the antenna with those drainage holes in any position other than downwards the chances are that water may have built up in the loading coils which has two effects. One is the obvious one, water equals corrosion! Also, if there is moisture in there, then it will alter the dielectric values between the loading coil winding, thereby de-tuning the coils or at least making them resonant well away from the desired frequency for any given band.

Another point is that you have been using a tuner to obtain a suitable match for your transceiver. With a wire antenna this would be perfectly fine, but if a tuner is used with a loaded antenna such as the Diamond CP-6 then there is a real danger that you could burn out the loading coils in the ground-plane radials.

When operated away from their natural resonant frequency, voltages much greater than would normally occur can 'puncture' and flash over inside the loading coils, causing either a permanent short (which would cause the loading coil to resonate at a higher frequency than usual), or sometimes a resistive short forms, which tends to make the tuning/setting up of the antenna very vague, and normally gives rise to a higher SWR than would otherwise been seen.

It is difficult to diagnose fully the problem from your post, but reading between the lines it could be a combination of the above, or it could be that the antenna simply is not tuned correctly as yet. However, with the six metre band working OK and all other bands running a high SWR it still points towards maybe water damage or coil burn-outs.

It could also be that the traps in the vertical section of the antenna are faulty too. Again, as only the six metre band is OK and all the others are failing, you might have a fault in the lowest trap on the antenna (i.e. gone open circuit), or at least you could possibly have corrosion at the joints in the tubing causing the top of the antenna to appear disconnected. This would also give rise to a low SWR on six metres but high SWRs on the other bands the antenna covers.

I would suggest doing a thorough Mk1 eyeball inspection of your antenna, dismantle ALL of the joints, clean and grease them throughout. Also test the DC continuity of the lowest mounted trap/loading coil on the vertical section of the antenna.

If you own, or know anyone with an antenna analyser, it might be worth testing the antenna with it, and try connecting directly to the base of the antenna to see what's really going on, before you reconnect your 15yr old coax!


Hope the above helps!


73, Mark.
 
Hi,
thanks for the interesting information you raised in the reply especially regarding the tuner
I have a flex3000 and as such need the swr down to 2.1 tu use the tuner as I cannot do this then ive used a welz matching network ac38m in between rig and antenna to get the swr down
Since my last post i read an article by yourself I think regarding the length of coax used
my total length was around 30ft so ive joined both cables together making a length of around 80-90ft. This lowered the swr right down to around 5.1 on all bands
and in fact less on others.!!
Im using the tuner to just get it down a bit to 2.0 so I can use the rigs tuner and this now seems to be fine
but worried about your comments regearding using the tuner.
The antenna is only a couple of months old so no corrosion, and the holes are facing down
Once again thanks for the intresting info and all comments are welcome.

all the best
Mike G0MRY
 
Hiya Mike

Glad to see your SWR is coming down!

As I mentioned in that other post on this site, feeder length really does make a difference, but I'm still not convinced that you would have had a 12:1 SWR on all bands with your original 30ft length. For example on 80m you might have seen maybe a 2.5 or 3:1 SWR, but not 12:1. 40m might have been out by a little more.

As you go even higher, because you then fall in to the realm of multiple quarter-waves, unless you were using REALLY low loss cable, you tend to find that the losses in the cable tend to 'mask' the problem (the cable itself absorbs the power!).

I agree that joining a number of sections of cable together would alter the overall phase shift along the coaxial cable, but it rings alarm bells when you say that ALL of the bands have fallen to around 5:1 (or thereabouts).

If you still find that six metres has a low SWR when compared to the other bands, that still points towards a fault in the antenna itself.

By joining multiple lengths of coaxial cable together, as the length increases so do the losses, so that again could cause the cable to 'mask' the problem.

To give you an example of what I mean I once had to use a long run of cheap (poor quality) feeder about 150ft long, and what I found was that I could measure an SWR of around 1.4:1 or less right across many of the HF bands. The thing was, the 1.4:1 SWR was obtained WITHOUT ANY ANTENNA FITTED, the losses were such that the cable was absorbing much of the forward and reflected power, so thereby giving a false indication at the radio end of the feeder.

I would still strongly suggest you try an antenna analyser if possible, connected directly at the base of the antenna, even if it's just for peace of mind.

A Diamond CP-6 antenna should easily give at least a 1.5:1 SWR when tuned and mounted correctly (at least 6-8ft off the floor), and in most cases pretty much a 1:1 SWR.

The downside with a Diamond CP-6 (or any other loaded antenna), is that the SWR tends to rise rapidly as you move away from the resonant frequency of the antenna. Yes, you can use a tuner (coupler would be a better term since it doesn't actually tune anything), but you will waste lots of power, both within the 'tuner', and throughout your coaxial feeder.

A bit more to think about there :cool:

73, Mark.
 
Hi
intresting points there
Ive not got any access to a analyzer so just have to try best.
Other thing was that ive dropped the cable to the floor now

When I first set up the antenna it seemed fine and I was operating from a room outside the house. the cable ran down to floor level around 15ft then into the shack
I then moved to a bedroom (warmer) and then ran the cable from the bottom of the antenna to the bedroom window more or less horizontal.

now ive extended the cable its dropped back onto the floor from the antenna then goes back up to the bedroom, do you think this would make any difference?

Is there any other test I can do without a analyzer?

all the best

Mike
 
Hiya Mike

OK, well, if you don't have access to an antenna analyser, why not try recreating the original conditions under which the antenna worked fine. At least you could see if the antenna really was OK.

Also, did you retune/readjust the antenna when you added the extra cable?

Have you added a good current choke under the antenna at the feed point? If the choke is too low in value you will get lots of RF current flowing along the coax shield which leads to erroneous/erratic tuning of the Diamond CP-6.

Have you made sure that the 80m radial is NOT adjacent to the 40m radial? (Instructions with Diamond CP-6 warn about this point causing problems with tuning).

If you did try to retune the antenna did you move the ends of the radials in and out by small amounts? Even just an inch or so of movement on the lower bands can move the point of minimum SWR by many tens of kilohertz, so it is very easy to miss the point where the SWR dips.

Re-reading your last post the main points that spring to mind are:

  1. 1 Lack of a good choke at the feed point allowing RF current to 'escape' along the outside of the feeder shield, so that as you altered the length and/or position of the coaxial feeder you inadvertently altered the tuning on the antenna.
  2. 2 Altering the length of the feeder altered the phase shift inherent in the feeder, thus giving rise to 'false' SWR readings (as mentioned in another one of my posts on this forum).
  3. 3 That one or more of the connections (in the coaxial cable) is faulty, giving rise to either a short on the feeder at some point, or maybe a lack of continuity on either the inner or outer conductors of the coax.
  4. 4 That the antenna itself is faulty, but you could only really prove this with adequate test equipment and known good cable/connections.

If you do not have access to an antenna analyser, it is possible to do a 'sweep' of the HF range by using a transceiver that has been 'expanded' or 'wide-banded' to allow it to operate outside of the Amateur bands, and reduced to QRP power levels (just enough to conduct SWR test).

Another old favourite is a GDO (Grid Dip Oscillator, or Gate Dip Oscillator). Use about a two turn coupling winding across the radio end of the feeder, and while tuning the GDO watch for the dips (indicating that power is being absorbed/radiated by the antenna). The dips on the GDO should be quite noticeable as you tune across each band the Diamond CP-6 covers. If they are vague, or even non-existent then the GDO is 'telling ' you that there is a problem!

Just have a look around the web for tips on testing antennas, and tuning them.

Reality dictates that in order to test things you need test equipment, and these days an antenna analyser is a sound investment, especially if you plan on moving your antennas around a fair bit, and also enjoy trying out different antennas. Analysers really do take the headache (and the heartache!) out of antenna experimentation, and they are one of the few bits of Amateur Radio equipment (maybe the only one!) that pay for themselves in the long run!


73, Mark.
 
Hi Mark,
Once again great info from you
NO I did not have a choke fitted at all so maybe thats a good starting point!

all your other suggestions are noted and will perhaps look at getting some antenna eqpt. (analyser)
will try the GDO (Grid Dip Oscillator, or Gate Dip Oscillator)as well so you have given me plenty to get stuck into.

Really got to say thanks for your time and help and wish everyone a happy 2010

all the best

Mike
ps will post report on my test when done
 
Very useful tips for me too - many thanks Mark! I got mine on long-term load from a friend and didn't know about the drain holes! Fortunately, just been to check mine and most of them were pointing down (more or less).

However - I still have a similar problem - I know that on 80m, the CP-6 is notoriously very tight but, no matter what I do to length of the element, I can't get it to tune better than 9:1 on frequencies above about 3.62MHz. Between 3.5 - 3.6, it is 3:1 or less but it then shoots up to 7 or more, settling at 9.1:1 on anything over 3.62MHz. Needless to say, as good as the IC-756 Pro tuner is, it won't help at that high a ratio!

All other bands tune beautifully, by the way - 40m, 20m, 15m, 10m & 6m are all sub 3:1 across the band!)

Any suggestions, gratefully received. (I need to be on 3.743MHz for the RSARS nets! :)

73,
Paul 2E0PAK
 
Hiya Paul

I know exactly what the problem is...😃

The standard resonator/ground-plane radial supplied with the Diamond CP-6 Antenna is designed only to work in the lower portion of the CW section of the 80m band.

There are two other radials (which are optional extras), one is called the "R-1 Radial", and the other is the "R-2 Radial". I've added a little info below...

R-1 Radial:
80m radial for 3.575MHz to 3.650MHz, for a better SWR in the upper CW/lower SSB range.

R-2 Radial:
80m radial for 3.650MHz to 3.725MHz, for a better SWR in the center SSB range.

The Diamond CP-6 Antenna relies heavily on its ground-plane radials being resonant to give the greatest efficiency on transmit (and receive too). I've heard a lot of Radio Amateurs using tuners (couplers) to try to force the Diamond CP-6 to operate on frequencies that it is not tuned for, the end result though is poor performance and possibly a burnt out antenna too.

The Diamond CP-6 is more of a spot frequency antenna rather than a general, full band antenna. For full band coverage antennas such as the Comet CHA-250b, Diamond BB7-V etc are better suited (assuming you can't use one of the large Butternut verticles that is).

To try and get around the problem of the rather narrow bandwidth of the Diamond CP-6 Antenna, Diamond have produced their new KV-5 model, which does away with radials though still uses the familiar trapped vertical radiator.

Hope the above helps!


73, Mark.

p.s. if you don't want to buy a new radial for the CP-6 Antenna, you could dismantle the one you have and remove some of the windings, or, you could add a temporary homebrew coil/inductor in parallel with the original radial loading coil so as to reduce the overall inductance.
 
That's excellent, Mark. You are a fount of knowledge on the beast that is the CP-6! 😃

As I mentioned, I have this one on loan, but with a view to potentially buying one myself. To date, everything about it is ideal, apart from 80m. Maybe, when I do buy one, I'll either get the R2 radial or potentially go for the KV-5 instead (assuming it's as good on the other bands as the CP-6 is).

Whilst not intending to never do CW (I am learning!), SSB is where I need to be at the moment.

Thanks, once again, for very useful info!

73,
Paul
 
I have the same problem, 6 metres is fine 1.2vswr and 10 metres is 1.8vswr but all other bands very high vswr 8-10
we spent the best part of the day trying to tune it to no avail

i can tune all bands fine with the manual atu, but surely this is not good for the antenna??

and have had good reports

but i know it needs tuning to its resonant frequencies

Those above how did you get on with the suggestions?

I am also going to see if i can borrow an antenna analyser to see whats going on

it has baffled me as to why it wont tune correctly

I thought it was because it was close to the house so we moved it to middle of garden and no change in vswr values hmmmmmmmm

even checked the connections changed from 360 to 180 configuration

even resoldered the RG213 Plugs

Cheers Guys
 
Hiya Lee

To recap the things that generally cause problems with tuning Diamond CP-6 Vertical antennas:

1) Water damaged or burnt out traps on the vertical radiator
2) Poor continuity around joints of traps
3) Water damaged or burnt out resonator/loading coils on the ground plane radials
4) Poor continuity around joints on radial loading coils
5) Antenna mounted too low
6) No choke in the feed cable
7) Unsuitable cable lengths
8) Incorrectly mounting the 40m radial next to the 80m radial
9) Damaged cable due to excess power or water ingress
10) Corrosion of the connector on the base of the Diamond CP-6 leading to poor continuity
11) Incorrect assembly of the CP-6 i.e. traps fitted in the wrong order
12) Tuning the radials too rapidly, only move them about a quarter of an inch (4-5mm) at a time, not in and out in three or four inch jumps! You will miss the correct tuning point if you do this.

With the Diamond CP-6 HF vertical you can often tell what the fault is by looking at what is going on with the tuning of the antenna.

In your case you say that 6m is fine, 10m the SWR is 1.8:1 (you should be able to get this lower on a 'good' antenna), but that all the other bands are poor i.e. high SWRs.

From the above we can deduce that at least the antenna connector and cable seen OK, and that there is a good connection to the six metre section of the vertical element up to the first trap. Because ten metres has a higher SWR that I would expect on a CP-6, and because all of the bands below ten meters have such a high SWR, I would suspect that possibly either the six metre trap (the first trap up from the bottom of the antenna vertical section) may have developed a continuity problem (check this with a multimeter, should only read one or two Ohms tops), or it may have burnt out windings (more difficult to check, best way is to substitute a known good one, or use an antenna analyzer. If windings are shorting out the antenna will resonate a little higher than expected on bands that rely on that trap for loading).

Else suspect the ten metre trap. Check out the ten metre trap for both continuity, (measure from end to end with a multimeter as before, should only read a few Ohms), or burnt out/shorting windings (as before, use analyser, all band that rely on the ten metre trap will either be resonant higher than expected, or else exhibit high SWRs).

Check ALL of the joints on the antenna, clean them if needed with some wire wool or abrasive paper, and wipe over with a suitable oil or grease to help slow down future oxidisation.

Make sure that the base of the Diamond CP-6 is at least six feet (two metres) above the ground or any nearby conductive objects i.e. tin roofs etc. Mounting the antenna too low is a very common cause for being unable to tune up the Diamond CP-6.

Always make sure you have a choke at the feed point, again a big cause of CP-6s not tuning up correctly is because they were installed without a choke, allowing the coax to become an active part of the antenna (RF currents not only flow along the radials but along the outside of the feeder and tend to 'mask' the correct tuning point).

It is also worth pointing out that sometimes the antennas are fine, but the SWR metering equipment may be faulty or out of calibration. The scenario here is that when you use your tuner direct in bypass mode the SWR appears high, but when you bring the tuner in to line you can then get a good SWR. Problem is that if the SWR meter is not calibrated correctly you may be 'tuning' for impedance of say 250 Ohms of some such. Worth keeping in mind anyhow.

The absolute worst case is that all of your radials have water damage in their loading coils, apart from the six metres one that of course has no coil.

If your Diamond CP-6 is mounted above six feet in height, with good coax, correctly fitted plugs, a choke at the feed point, and no burnt out radials, then I would still suspect either the six metre or more likely the ten metre traps to be the cause of your problems.


Hope the above ramble helps,

73, Mark.
 
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