Measuring antenna SWR w/ a RLB & SA - Page 1
Forum section: electronics / rf-microwave

author: videobruce
author_rank: Frequent Contributor
timestamp: March 04, 2021, 07:24:08 pm
content: To clarify the procedure to measure reflection/SWR from an antenna using s SA (Spectrum Analyzer) with a TG (Tracking generator);1. Connect the RLB (Return Loss Bridge) to the SA and set your frequency range,2. Turn on the TG and normalize the scan without any load on the RLB.Is that it or should there be a dummy load on the DUT (antenna) port of the RLB? That part isn't clear in all the places I have read about this.I also did a scan with the DUT port 'shorted' by a N connector I made, but the differences were minor vs leaving that port 'open'.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 04, 2021, 07:31:19 pm
content: As to an result of a actual antenna scan, here is a discone that doesn't look 'right' to me. The feedline is around 50' of Shireen RFC400 w/ N connectors. Nothing in between. The line was 'swept before it was installed.There is a pic of this discone of unusual designed below.

author: cdev
author_rank: Super Contributor
timestamp: March 04, 2021, 07:49:34 pm
content: That is not a discone but its similar to one. Its more like a biconical antenna but I would expect it to have issues as a biconical too. I see why it was constructed as it was, its hard to make a biconical antenna properly without a quite decent metal shop.Also, your long length of coax may be impacting your measurement quite a bit. Better to measure it without a long cable like you are. The cable will make the SWR look better than it really is, I'd guess.Is that a commercial product?You could use a nanovna to measure the SWR very quickly. It is the equivalent of a siggen and a tracking generator and more.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 04, 2021, 08:04:02 pm
content: "commercial product", the antenna?Not really, thou I guess they probably consider it one. Then a longer feedline 'smooths out' the result?Ok, how about this, it's a VHF 'Ringo' note the 'nulls'.;

author: cdev
author_rank: Super Contributor
timestamp: March 04, 2021, 08:11:42 pm
content: The dips are points where the antenna is more resonant. They probably map to the discontinuities represented by the bends and the tip of the wires.Yes, definitely. I would expect a long length of coax attached to make it more like the characteristic impedance of the feedline. Very low quality coax can make an SWR look better.If you have a swr measurement setup that includes a tracking generator thats sweeping over and over do this as its running take your finger and run it down the wires see if you can see any changes as you touch different spots, its quite possible that by doing this you'll be able to figure out why they put the bends where they did by doing that exercise. Its very informative. The antenna you have appears to be an uncomplicated, fairly easy to construct antenna that probably costs almost nothing in materials but still fetches nearly as much as a discone, especially if its marketed skillfully.

author: hendorog
author_rank: Super Contributor
timestamp: March 04, 2021, 08:31:58 pm
content: Quote from: videobruce on March 04, 2021, 07:24:08 pmSounds like you are doing it right to me. The Open (i.e. Port open) and Short standards are theoretically equivalent in your system, as they theoretically only differ in their phase - and your scalar setup doesn't measure phase.Any differences are just due to the physical differences between the two standards.A scalar network analyser (i.e. SA/TG) has limited 'dynamic range' when measuring return loss due to the limited directivity of the internal bridge which it can't correct for. When you measure an antenna at the end of 50 foot of cable, the loss of the cable 'eats up' some of that range on the way out, and again on the way back. Since you only have so much to play with you haven't got much left to measure the antenna.So the advice to measure the antenna with a much shorter cable will help, particularly at those high frequencies. You can see for yourself how much this matters in your case by disconnecting the antenna from the feedline and attaching a good load on the end instead. Then measure that - feedline + load and see what it looks like. Could be approaching 10dB for that cable including connectors.You can also determine how well your system is working by measuring the load directly on the measurement port after normalising. If the load is near enough to perfect then that will tell you what your lower limit is.Measurements close to the limit will be significantly less accurate. There are good charts on the www about this.You should normalise with the port open (or shorted) before doing any of this.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 04, 2021, 09:33:22 pm
content: cdev;That last scan is a VHF 'stick' tunes to 159 MHz. The ones in the UHF portion, I have to assume are 'quirks' since that is a VHF only antenna for a specific frequency which is extremely hard to tune from the lack of de-coupling from the mast (no ground plane).hendorog;Attached are scans with 2 different 'loads'. One being a N terminator, the other being a 1 3/4 x 1 3/4 x 4 1 1/2" dummy load, I'm guessing 50 watt.

author: xmo
author_rank: Regular Contributor
timestamp: March 05, 2021, 12:46:33 am
content: It's normal for a termination to have better return loss than a dummy load so your results look fine.For testing an antenna, there will typically be a specified SWR across some range of frequencies which you can verify.Return loss results are degraded by twice the coax loss so take that into consideration when evaluating an antenna.Here is a return loss sweep of a DB-224 VHF antenna.Homework assignment: Why the delta marker measurement on the return loss ripple and what does it tell us?

author: videobruce
author_rank: Frequent Contributor
timestamp: March 05, 2021, 03:42:20 pm
content: It was suggested to disconnect the feed line to the antenna, normalize the line to remove the effect on a long feed line vs a short 'jumper'. Then re-attach the feed line and test. Does that sound right?Wait a minute, a "homework assignment"??You got me, I give up.

author: xmo
author_rank: Regular Contributor
timestamp: March 06, 2021, 05:19:20 am
content: If you had someone climb the tower to disconnect the feedline, and then you normalized the instrument, you could remove the feedline loss from the display.Once the antenna is re-connected, the ripples on the trace will still be there because they result from the mismatch at the antenna. Actually, that's the reason for measuring the distance between two peaks on the ripple. The peaks are one half wavelength apart.Once you know the frequency difference between two peaks, you can calculate the length of the transmission line:L = Vp x (SOL/2xFd) where:L = LengthVp = Velocity of propagation in % of free spaceSOL = Speed of LightFd = Frequency differenceSolving for the example where Fd = 1.1 MHz and Vp = 89% for Andrew 7/8" Heliax yields:Length = 400 feetContinuing the analysis, the coax loss is 0.45 dB/100' or 1.8 dB/400'.So, twice the coax loss would be 3.6 dB and the actual return loss of the antenna would be 3.6 dB poorer at any display point.The antenna being swept is a DB-224 which is specified to have 1.5:1 or better SWR across a 10 MHz bandwidth. 1.5:1 SWR = 14 dB Return LossThe display indicates that the antenna meets it specifications.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 06, 2021, 10:20:19 am
content: That's well over my head.The good news is, there is no 'climbing a tower', just a short roof ladder to the peak thru a 'roof hatch' via a stepladder in a (actual) attic.

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2021, 09:23:30 am
content: UNCAL is a warning you have changed settings after normalizing.Ideally you need to make settings based on best judgement, maybe centered on the DUT expected frequency then Normalize then connect the DUT and examine the sweep and maybe even View/freeze it.Change settings to better display DUT parameters and select a second trace if judged necessary then Normalize the new settings and connect the DUT.If you have the Return Loss option installed you might wish to use that instead of the normal SA settings.Grabbed from a Siglent webpage:

author: videobruce
author_rank: Frequent Contributor
timestamp: March 07, 2021, 10:53:32 am
content: The 'uncal' is because the RBW & VBW are set to manual.But, I would still have to use the RLB with that SA option, correct?

author: xmo
author_rank: Regular Contributor
timestamp: March 07, 2021, 03:19:08 pm
content: "Uncal" is warning you that your results may not be valid.There can be times when setting one or more parameters manually improves results.Manual setting does not mean the result is uncalibrated, only a combination of manual settings that produce potentially invalid results will activate the "Uncal" indicator.It's telling you to choose another combination that is valid. A valid manual setting combination will not show "Uncal"

author: videobruce
author_rank: Frequent Contributor
timestamp: March 07, 2021, 07:12:11 pm
content: I've put those back to 'auto' but it didn't really affect these scans all.

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2021, 08:09:14 pm
content: Reply #3 sweeps are what I would initially expect to see and then adjust settings and Normalize again for a better look at what's going on.You should be getting results like xmo displayed once you have the right dB/div scale in place.I'm of the opinion the feedline is part of the system and whenever possible measurements should include it.Recently sold my SSA3032X and RB so sorry can't really play along at home however a couple of threads about antennas might offer some tips or guidance:https://www.eevblog.com/forum/rf-microwave/what-really-is-this-antenna/https://www.eevblog.com/forum/rf-microwave/antenna-project-log/