A quick Standing Wave check on my antenna system (baselining)

A quick check of my antenna systems SWR (VSWR) tells me that I should be able to use the antenna system on five AR-Band without to much trouble. However a few quick note before heading of to the actual task.

I'm not going to talk about impedances, reactance or admittance. I'm simply checking the SWR to get a basic overview of the overall antenna-systems ability to be used with with my transceiver and/or being able to use a small ATU (Antenna Tuning Unit) to pretend the SWR is "good". It shouldn't really be known as an ATU as it really isn't tuning the Antenna. It is a device using lumped circuits (L's & C's) to present a match to the transceiver output stage. Which is not a constant 50Ω at any of the AR-Bands either. So this quick check of the SWR is enough information which tells me all I want to know (at this stage). 

An additional note. You might have noticed that I always say check and checking the SWR! Well I've never seen a SWR meter that measures SWR (how would we be able to measure a ratio). SWR is not a measurement it is a calculation! Generalised, a VSWR (Voltage Standing Wave Ratio) check is a Voltage measurement of the forward and reflected voltages at one frequency. From those two values the SWR is being calculated. 

So onto the antenna, it is a 40m horizontal loop, attached to two TV roof standoffs to clear the edge of the roof and than slopping into the backyard to a height of 2.3m 5m above the ground.

It is feed with about 3m commercial 450Ω  ladderline to a 1:1 current balun. The rest of the feedline is about 20m of LMR400 and a short run of RG213 and RG8X for the interconnections between the ATU, AMP and the Transceiver. The feedline is heavily chokes with homebuilt chokes.

Measuring from the 213 I'd say I've got 23m of 50Ω feedline to the balun and about 3-4m of 450Ω feeder. My guess is (but I should really measure it) that I don't have to worry about too much loss through the feedline on the four HF-Bands and even on 6m the line should not be to lossy (not sure about the BALUN though, more checking/measuring required).

So how does the SWR look like. (NOTE: If I talk about the SWR from now on, I'm talking about the antenna-system SWR and not the antenna SWR.)

I do not use an inline SWR meter for this purpose, the inline SWR meter is, and that is my believe, only good for monitoring if a change in the antenna system has occured. 

For this tasks and long term comparisons (baselining) I'm using a RigExpert antenna analyser. 

Basically I'll check from after the ATU, i.e. from the end/beginning of the RG-213 upto the antenna.

Below is a picture of the result.

It only displays the five bands that have a reasonable SWR. So let's zoom in a bit.

1. 40m

Bit low in the band, my aim was 7.100 but I thought this wasn't to bad straight of. It shows the VSWR at 40m is good to very good, with an average SWR below 1.5:1.  

The other bands have their best  VSWR outside our allowed frequency allocations. And experience tells me that I'll be able to use my ATU to present an acceptable SWR to the transceiver for proper operation of the output-stage. But even on 40m I should use an ATU to keep the transceiver/amplifier happy as the above 40m SWR plot clearly shows.

2. 20m

On 20m the situation is not as bad as it looks, best SWR is around 13.9MHz. But we also can see that the SWR is not to bad across 14-14.35MHz. With max SWR of less than 4:1 at 14.35MHz. Yes, looking at the Z, e.g. the impedance, I would be able to see if my ATU would be able to tune that. But for now this is all I need.

3. 15m

On 15m the situation is very much the same.

4. 10m

On 10m however, the SWR bandwidth is quite broad and in most cases I'd not need to use an ATU unless I go into the FM spectrum.

5. 6m

And last but not least the bonus Band, 6m. The spectrum I'm mostly interested in, 50.1-50.4MHz has a VSWR greater than 2:1 and would need an ATU to keep the transceiver happy.

Now all this means is that I should be able to operate on these bands without to much trouble. The ATU's build into the newer type of Radios and Amplifiers with their 3:1 tuning range should find a suitable match without breaking sweat. And, my trusty old YAESU FC-901 ATU is able to tune the four HF bands easily without getting warm at 400W.

So now that I have this data "stored" I can say that I have baselined my antenna system. I can now refer back to this data to see if, over time changes have occurred.  

Last thoughts:

• For an antenna SWR the SWR should be checked at the antenna itself rather than at the end of the feedline. The feedline will load the antenna and create an illusion of having a better antenna SWR.
• To fully understand your antenna-system, feedlines (transmission lines) should have their attenuation (cable loss) measured or calculated (length measurement tape-measure  or TDR).
• Knowing the above, one can calculate the SWR at the antenna feedpoint.
• I would not use an inline SWR Meter for any of the above measurements however, using an inline SWR-meter is good insurance policy because connection problems usually show up as SWR spikes which can quickly be seen on those type of meters during operations.

My transmitted signal analysed

 A quick look at a spectrograph to analyse my transmitted signal.

Setup:

Transceiver : ICOM IC-7610

Power       : 108W

Antenna     : 500W dry dummy-load with 40dB tap

Modulation  : SSB (LSB)

Audio setup : see here

Receiver    : RF-Space SDR-IQ

Software    : SpectraVue Ver.3.39

IF-Gain     : +6dB

RF-Gain     : -20dB

Lets analyse the signal.

The grey section shows the receiver audio bandwidth (BW), i.e. the SSB RX filter bw or the CHANNEL BW. 

The black area is the so called Video BW, in this case a 30kHz view of the spectrum. A frequency spectrum display from 7.160MHz to 7.190MHz .

The green line is the real-time capture and the blue line is the in-time capture, the memory of the previous capture. 

The display also shows some vertical lines with a distance (spacing) between two lines of 10dB and there are also some vertical dotted lines which are 3kHz apart. Additionally I have applied "smoothing" to quieten the real-time capture. (I'll show the same signal without smoothing further down below) 

Now let's look at the BLUE line (the Sideband envelope), as that line dipics my previous SSB transmission. The top of the signal is about -59dB (S9+14) a reasonable strong signal. We can also see that the width at the top of that line fits into the grey, the 3kHz bandwidth marker, and is about 2.9kHz wide (100-3000Hz). We can also see that the signal slowly spreads out towards the bottom, down to the about -90dB from were it then spreads between a bit more quickly to 7167.8kHz and 7182.8kHz. Since the carrier frequency is at 7175kHz, we can deduce that the signal is spreading -7.2kHz and +7.8. 

Struth, what a crappy wide signal ! 😇

BUT, wait! The WIDENING of the signal starts at approximately at -91dB (S6). Which would mean that from the peak of the transmitted channel BW the signal is "clean" for about 32dB. Now that is a pretty good NON pre-distorted signal. 

The following table shows how the signal from about the -90dB mark spread very quickly, but also drops very quickly in signal strength. 

Frequency	Signal Strength		Frequency	Signal Strength
7169kHz	-100dB	S4.5	7169kHz	-100dB	S4.5
7166kHz	-114dB	S2.2	7181kHz	-114dB	S2.2
7163kHz	-124dB	S0.5	7184kHz	-124dB	S0.5

NOTE: S-Value steps are in 6dB

This does help me to understand how wide my signal really is and when and how I would cause channel interference based on a 3kHz channel spacing. 

Below is the same signal without smoothing.

As we can see it is very easy to check our own transmitted signal. The requirements are not that strenuous. A  tap, dummy load and a SD-Receiver is all that is required to not only check our transmitted audio, but also our transmitted signal. Here are a couple of tap's that are easy to replicate, a 40dB and a 50dB tap.

A Splitter/Combiner

A while back I build a combiner for some two tone tests on some older type transceivers. Well the transceivers have now gone and so I found new use for the combiner as a splitter. 

The combiner has pretty good port isolation of over 50dB and an insertion loss of about 6dB. To test some of my SD-Receivers I use the splitter to divide the receive signal from the antenna and feed two SDR's so I can compare rx-prowess. 

The toroid I used is my trusty old workhorse, a Jaycar Model LO1230 (with the dimensions of 18x10x6mm) which has very similar characteristics as Mix 43. A FT50(A)-43 might do a slightly better job (maybe), but the LO1230 is available locally at a reasonable price and does the trick nicely. The resistors are 50Ω resistors (green gold white white yellow) (I have a few left overs from other projects) but a 49.9Ω 1% resistor should do the trick as well. 

Now, the 100nF capacitor is more or less an insurance policy, it is used as a DC blocker. You could leave it out if you so desire. I've put 12 bifilar windings around the toroid and that's it. Nothing to elaborate. 

Here is a quick sketch of the unit:

And here are some measurements of the unit.

1. Attenuation:

2. Port isolation:

As can be seen the isolation is very good from 80-6m (3.4MHz - 50MHz) but is still good enough for rx testing below 3.4MHz and above 50MHz. I might test again to see what the specs are for below and above frequencies. Anyway as it is it works for me at the current use, which is being used as a splitter and not a combiner.

3. Photo of build unit:

I currently have the unit setup for my SDR-IQ and RSPdx to compare rx prowess at ELF, VLF and HF using SDRConsole, SpectraVUE, SDRuno and HDSDR. The 6dB attenuation is only a problem at the higher end of the spectrum which I can compensate with an assortment of low noise amplifiers (LNA's) from Minicircuit and homebrewed units.