Hi
I purchased three NL-770R antennas of ebay a number of years ago fro China. Over time two of these failed due to the internal capacitor in the base breaking so I decided to order three more (this time from Singapore).
When the new ones arrived all three had excessive VSWR and were unusable. They looked exactly the same as the ones I had previously purchased. After some email ping pong with the supplier I was refunded for them by eBay.
As I was still after the antennas I ordered two more in two separate orders this time from an Australian supplier on eBay. These arrived this week. They looked the same as the others but used soft steel for the elements. They also were unusable due to excessive VSWR.
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Existing NL-770R antenna VSWR 146 MHz 1.5, 435MHz 1.4
The ping pong game with the supplier has been started today.
Problem is I still need to source dual band antennas no longer than about 1M so they fit into the garage. I will just have to keep trying!
73s
Igor
Existing NL-770R
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Igor
Igor
That may not be a fair way of testing them. They are a mobile antenna so will normally operate with a ground plane and sitting them atop an antenna analyser will not provide the same earth plane effect. It will certainly screw up any feed impedance measurements.
Screw them onto a mobile base properly mounted on a vehicle and re-measure at about the same frequencies - as well as sweeping to find out what the SWR minima frequencies are. Post those results too so we can compare for ourselves. Numeric format is fine, rather than the series of photos...
Adjusting the radiator lengths telescoping into the holes (i.e. by releasing the grubscrews) move the results much ?
{ base grubscrews mainly for 70cm, ones above the loading coil more for 2M, but both interact }
Doug
(13-09-2019, 07:28 AM)VK4ADC Wrote: [ -> ]Igor
That may not be a fair way of testing them. They are a mobile antenna so will normally operate with a ground plane and sitting them atop an antenna analyser will not provide the same earth plane effect. It will certainly screw up any feed impedance measurements.
Screw them onto a mobile base properly mounted on a vehicle and re-measure at about the same frequencies - as well as sweeping to find out what the SWR minima frequencies are. Post those results too so we can compare for ourselves. Numeric format is fine, rather than the series of photos...
Adjusting the radiator lengths telescoping into the holes (i.e. by releasing the grubscrews) move the results much ?
{ base grubscrews mainly for 70cm, ones above the loading coil more for 2M, but both interact }
Doug
Hi Doug
Tests were carried out this way to make it as simple as possible as I did not want to get into a long protracted ping pong session with the supplier which has now started.
"We apologize for any inconvenience this has caused to you.
How about we refund you AU$ 4 as a compensation? Maybe you can repair it in your local shop.
Because we also suffer loss, we paid the shipping fee and the eBay final value fee. You can keep it and do not need to return.
After the refund, if you are satisfied with our service, please close the case and give us a positive feedback.If not, please kindly not leave us any neutral or negative.
Waiting for your reply and Have a nice day."
Similar results are obtained when the antennas are tested in the vehicle installation. However this is more difficult to show convincingly in a photograph that the antenna being tested is actually connected to the test gear.
Adjustment of lengths does not enable the VSWR to be brought to an acceptable level. This was tried. This only works if the antennas are close to being in tune in the first place. Being dual band an improvement in the VSWR in one band is usually detrimental to the other. The matching unit at the base of the antenna is way to far out of tune for length adjustment to work.
The antennas are of the end fed nominally ground independent type. Ground independents are not really totally ground independent. They actually require a nominal counter poise for correct operation. All electrical circuits require two connections though this may not always be obvious as the coax feeder outer may provide this. The counterpoise needs only to be about 0.05 of a wavelength from memory. The VSWR test unit size is adequate on UHF and marginally on the small size on VHF. To validate the test procedure I checked by adding counterpoises to see if this made a difference. It was only very marginal on VHF.
I did carried out VSWR return loss sweeps on the previous set of faulty antennas from 100 to 500 MHz using my Rigol DSA815 but these are in dB. I did not go down this path as they would confuse the supplier as the published data for the antenna is quoted in VSWR.
73s
Igor
PS
For reference previous batch of antenna minimum VSWRs for in vehicle installation were (disregarding other odd frequency mininums)
Antenna 1
VHF VSWR min 2.6 between 141.3 - 142.3 MHz
UHF VSWR min 1.4 between 439.7 - 442.7 MHz
Antenna 2
VHF VSWR min 2.3 between 140.5 - 140.7 MHz
UHF VSWR min 1.6 between 438.6 - 441.7 MHz
Antenna 3
VHF VSWR min 2.2 between 140.6- 141.4 MHz
UHF VSWR min 1.5 between 438.6 - 443.1 MHz
Existing NL-770R
VHF VSWR min 1.0 between 146.6- 147.3 MHz
UHF VSWR min 1.1 between 437.5 - 440.0 MHz
Not wanting to start another brand war... I tried a Nagoya (that I still use at times especially when I am concerned about someone stealing it off the vehicle) and have a number of Diamond antennas of varying lengths (that I use more often).
If you go here
http://www.diamond-ant.co.jp/english/ama...o5_nr.html you can see the sizes of the Diamond antennas, then go to Strictly Ham to get the price.
I had a similar issue with overall height due to the underground carpark at work a few years back and more recently when I have visited people in hospitals. If I forgot to swap the antenna I would hit fire service pipes, fluro lights and signage as well as scrape the tip on the roof in some spots.
Driving a larger 4x4 means that the base is quite high to start off with.
Regards
Bernard