EMRFD Message Archive 10985

Message Date From Subject
10985 2015-04-20 02:59:13 Kirk Kleinschmidt Will the Real SWR Please Stand Up?
Hi, gang,

Long Post. Please offer insights. :)

I am a prime example of the adage: "If you give a cave man an electric guitar, don't expect to hear Hendrix anytime soon..."  :)

Nevertheless, I'm in the toddler stage of getting on 2 meters and 70 cm for the first time in 38 years as a ham, and I'm learning how to use some exciting -- and confusing -- new tools, namely a MiniVNA and a Rigol DSA815 spectrum analyzer.

I built a four-element, 2-m Cheap Yagi per WA5VJB's design tutorial, which I installed in my attic on top of a TV rotator. My 40-meter horizontal loop runs around the perimeter of the attic and is tuned with an attic-mounted autocoupler. As shown in the photo, the rotator is on a "sled" so I can deploy it and retrieve it from the attic access hatch without having to actually get into the attic itself. See photo:

I tuned and checked the antenna while it was mounted in the yard (about 6 feet above ground) on a non-conductive mast. The first tool I used to determine the initial SWR curve was an MFJ antenna analyzer (borrowed from a friend). It put the low-SWR point at 144.83 MHz, 1.2:1. There were a few unexpected dips below 144 MHz, which I ignored at that point:)

The next evaluation tool was my MiniVNA, which showed the low-SWR point at 145 MHz, 1.42:1, 15.2 dB return loss, which is pretty close to the MFJ. See photo:

Next up was my Rigol DSA815 spectrum analyzer. I didn't happen to have a "real" directional coupler/return loss bridge, but WA5VJB himself suggested that a 10-dB CATV tap/directional coupler, even though it was designed for 75-ohm circuits, would still give useful results with my gear. My local CATV jobber had several and, since they were less than $5 each, I bought three. The one shown here is a Blonder Tongue 9-dB tap/directional coupler. Two other models, a MACOM and a no-name, had more variation between "return-loss mode" and "advanced SWR mode," so I limited the results for the time being.

Coupler/SA Setup. As shown on several youtube videos, I connected the tracking generator output to the "out" port, the SA input to the "tap" port, and the test antenna to the "in" port. I would have never figured that out on my own... I used 1-foot-long RG-6 cables for the runs between the coupler and the SA. I have since acquired a Mini-Circuits 20-dB directional coupler, but because it uses Type N connectors, I don't yet have 50-ohm jumpers with which to connect it (soon).

I know there are a lot of variables here, but here goes. In Return Loss Mode with the Blonder Tongue directional coupler, normalized with the coupler inline but the antenna disconnected, the low-SWR point is shown as 145.8 MHz, -28 dB return loss.That's nearly 1 MHz higher than the other devices, and I suspect that the return loss figure is too fantastic, although I may need to subtract the inherent coupler loss, etc, which is one of the things I need help with. See photo:

In Advanced SWR mode, with the same Blonder Tongue coupler inline, but the antenna disconnected per Rigol instructions, I ran an "open calibrate" procedure before connecting the antenna and pressing the "VSWR" button. This produced a low-SWR point of 146.05 MHz, 1.13:1 SWR, 23.9 dB return loss. Out of the three CATV directional couplers I tried, this is the one that produced the most consistent results between SA modes, but there's still quite a bit of variation. See photo:

When measured from my shack, which is on the other end of 50 feet of RG-6, I get this with the MiniVNA, which seems encouraging:

When I do a wideband sweep, however, I see a very "wiggly" SWR curve, which may be because the antenna is in my attic and is interacting with "stuff" in its proximity. Is this weird? See photo:

I also noticed that my horizontal HF loop, when measured with the MFJ analyzer at the remote tuner output balun, has a 1.2:1 SWR dip at 143 MHz! That may cause significant interaction?

As Wes says, we need to get out there and measure stuff :)  I am very interested in doing that, but I clearly need to learn a few things when it comes to using advanced tools. I have wanted a spectrum analyzer since I was a teenager. Now that I have one, I have to figure out how to use it. :)

I know that the 1-foot RG-6 jumpers between the RL bridge and the SA may affect measurements, and I know that the 9-dB CATV tap may tweak things as well, but I don't understand the differences between the MFJ/MiniVNA vs. the SA, and I don't understand the different results produced by the same Blonder Tongue 9-dB tap when used on the same SA, but in "standard" mode and "advanced SWR" mode.

I haven't transmitted through the antenna yet, nor have I checked the SWR on the radio itself. I suspect that the antenna will work okay, but I'd like to get some expert feedback on what I've observed so far.

I started to evaluate the 432-MHz Cheap Yagi with the "real" Mini-CIrcuits 20 dB directional coupler, but rain forced me to hustle the Rigol to safety :)

As usual, thanks for your help. I appreciate it.

--Kirk, NT0Z
My book, "Stealth Amateur Radio," is now available from
www.stealthamateur.com and on the Amazon Kindle (soon)
10986 2015-04-20 10:39:26 Phil Sittner Re: Will the Real SWR Please Stand Up?

You have some great instrumentation to take advantage of. You likely are aware that two feet of feedline at 2 meters represents a significant portion of a wavelength and transmission lines do act as impedance transformers. You may wish to investigate the effect of the 75 ohm line by using some Smith Chart software. I recommend and use SimSmith. It is quite intuitive and there are some great tutorials available that will quickly get you up to speed. Best of all it's free. Keep up the experiments.
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10987 2015-04-20 11:18:00 Kirk Kleinschmidt Re: Will the Real SWR Please Stand Up?

Regarding the coaxial jumpers between the directional coupler and the spectrum analyzer: I see that some microwave couplers have built-in connectors that connect directly to the spectrum analyzer, making for a very short (or zero-length) connection between the two. I don't have Rigol's matching coupler (spendy), so I'm wondering if I should make a set of jumpers that are either really short (for the band I'm testing at), or are a multiple of an electrical even half-wavelength for the band I'm testing?

Might that help eliminate any impedance transformation that takes place in those connections?

Or will using 50-ohm cables "take care" of any problems?

Or, will switching the spectrum analyzer to 75-ohm mode help? I should have tried that during my initial run... :)


--Kirk, NT0Z
My book, "Stealth Amateur Radio," is now available from
www.stealthamateur.com and on the Amazon Kindle (soon)

10988 2015-04-20 12:21:46 Phil Sittner Re: Will the Real SWR Please Stand Up?

It's common practice to use transmission lines for antenna matching purposes. For example, if your antenna has a 100 ohm impedance and you wish to feed it with a 50 ohm feed line, use a quarter wave section of 75 ohm line to transition from the antenna to your 50 ohm coax. There's a great section in the ARRL antenna book that discusses transmission lines and their use for matching purposes. You should also consider using a return loss bridge for measuring your SWR with your Rigol SA. On page 7.22 of EMRFD you'll find the details for building one. I made mine for less than 5 bucks and made the enclosure from double sided PCB material. It's useful to UHF frequencies. You'll be able to determine that with the Rigol if it's equipped with the tracking generator option. Your VNA can also measure it. Don't discount my advice about SimSmith. You can very quickly determine all types of things with a Smith Chart and if you are at all serious about antennas you can't do without it. Your transmission line system should always be of constant impedance unless you with to "transform" the characteristic impedance.
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