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.