Diamond D190

[G7RUX]

Evening all,

I have a Diamond D190 up on a pole for general monitoring at a work location. It’s not an ideal location but should work reasonably well. However the specs say it should give a reasonable match on 2m (apparently 2:1 or better) but I can’t see a return loss of any better than around 5.6 dB which equates to a little over 3:1.

Does anyone have any pointers as to what might be the issue here? Feeders are fine, installation is solid and I see the right sort of trends in the return loss across frequency so I’m a little at a loss as to what is going on here. The manufacturer’s blurb indicates it should arrive with a decent match on 144 MHz but trimming might be required so I may pop a bit of wire in place of the whip to have a play with trimming but I was expecting to see a better match on 2m as-is.

 

[Ubbe]

The discone part of the antenna are designed for 300Mhz and up and you have some good measurements there. The top whip are then supposed to support one additional lower frequency that might be 50MHz as there are no evidence in the graph of any sort of resonance at lower frequencies down to 100MHz.

Sometimes the rod are secured with an insex screw to the bottom coil and then you can try to use an electrical wire cut to different lengths to see if you get a better match at 144MHz and then cut the steel rod to the same length. One problem are that the coil are tuned to 50MHz and the remaining element could then be very short at 144MHz making the antenna very inefficient.

If needing a "real" 2m antenna then a $10 diplexer can be used that split up the signal below 250MHz and above 250MHz to allow a 2m antenna to be used together with the discone, and completely remove its top whip.

10W VHF UHF Diplexer Filter Band 2M/70CM Place Between Transmitter and Antenna | eBay

1pcx 10W VHF UHF Duplexer Filter to place between transmitter and antenna. Power: 10W(MAX) under the best matching condition. Band: 2M,70CM.

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/Ubbe

 

[G7RUX]

Indeed, I understand the design principles here but I am perplexed about Diamond’s claims…they state in the documentation the the whip part is delivered trimmed for 144 MHz (claimed VSWR is 2:1 “or better”) and indicate that trimming may be necessary to bring the tune up from there. However this isn’t what I see at all and the NA plot supports this. I had already spotted the inset clamp screw and intend to see if I can improve matters a little for 2m. It doesn’t need to be in any way superb as I have other feeders I could hang a dedicated 2m antenna from if it becomes necessary but I’m just perplexed why I’m not seeing close to a reasonable match on a claimed frequency.

I guess I am really asking if any other users of the D190 see this behaviour or have I put this admittedly simple antenna together in some way incorrectly?

 

[Ubbe]

The coil and whip length looks to be the exact same as used with their other discones, their 35-50MHz whip. They probably incorrectly use the same text as for the their full size discones. Try 1/3 of the whip length using a wire and see it you get any kind of change from the SWR sweep. The coil can have too much inductance to make that work.

/Ubbe

 

[G7RUX]

The coil and whip length looks to be the exact same as used with their other discones, their 35-50MHz whip. They probably incorrectly use the same text as for the their full size discones. Try 1/3 of the whip length using a wire and see it you get any kind of change from the SWR sweep. The coil can have too much inductance to make that work.

/Ubbe

Thanks, I was already thinking along those lines but am surprised that Diamond would get it so wrong.

 

[G7RUX]

I mean, this is really for short range monitoring of a system around 800-900 MHz so it works very well for the intended purpose but I had hoped to be able to use it for a smidge of 2/70 during down-time.

 

[G7RUX]

Well, that's tiresome.

I had a reply from my query to Diamond about this where I had asked what I might have done wrong; they were very certain that the antenna should work pretty well on 2m as the vertical is tuned as a slightly loaded quarter-wave vertical, which makes sense from the dimensions.

So I went back up and took the antenna off its mount, brought it back down and tested it on the analyser and it looked absolutely fine with a reasonably good match on 2m. Headed back up and popped a load on the feeder, puffed it from the bottom and it looked exactly the same as the previous plot which perplexed me quite a bit!

Digging around I found that the bundle of cables have been mislabelled, with 6 and 9 interchanged so I have been operating on the wrong feeder! Switch things around and voila! everything looks much more sensible.

In related news, an unrelated system which was performing poorly is now actually attached to its antenna and so suddenly started working!

Sigh.

Back to normal service!