A model of my 40m Delta Loop

At my QTH it is rather difficult to have dipoles strung up at a decent height. As I have had quite good success with horizontal loops (see my old Lazy-D) I decided to install a slopping Delta Loop at this QTH. From what I can tell the antenna is working quite nicely and I am able to make the radio happy with the internal tuner on the following bands, 40, 20,17,15,12,10 and 6m. Now it is not a WONDER antenna however, it does the job for me (YMMV).

Installation details:

• Feed point is 14m above ground (1.5m hockey stick above roof).
• The end corners are about 5m above the ground.
• Feedline, the first 3m are a commercial Ladderline (labeled 450Ω but it might be closer to 390/400Ω), then 20m of LMR-400, 3m of RG213 and 0.8m of RG8 from the ATU.
• The antenna is strung to face towards the East, at approx 110°.

As I also use this antenna as a RX-antenna for ELF/VLF I have removed the BALUN and only use a few chokes on the coax-feedline. Below is a drawing of the antenna.

40m radiation pattern:

As we can see, nothing really to talk about it. Pretty good for NVIS and with a bit of luck, e.g. very good to excellent propagation I might work a little bit of DX.

20m radiation pattern:

On 20m we can see two lobes. One to the East with a nice bit of low angle gain into the East which would help to work short path into the Americas and long path into Europe. However, the high angle lobe seems to be also quite good.

17m radiation pattern:

Even though the VSWR on 17m is not that great, the inbuilt Auto Transmatch (ATM) manages to get the SWR down enough for the Radio and Amplifier to be happy. Losses due to SWR are low enough and don't cause any issue. Looking at the pattern we can see that we have very similar coverage as for 20m. And, as it is for 20m, there is still a high angle lobe.

15m radiation pattern:

Now on 15m we can see that the high angle lobe has disappeared and that there are quite a few usable lobes. There are East and West lobes, with the East lobe quite low and strong, but also quite narrow. Additionally the North and South ones are quite broad with a nice bit of gain.

12m radiation pattern:

As for 17m, the VSWR on 12m is not that great. However, the ATM is able to keep the Radio or Amplifier happy. SWR losses are nothing to worry about. The radiation pattern is very similar to the 15m pattern with noticeable nulls in the North and South lobes. Again, lots of good gain into the East and reasonable gain into the other hemispheres.

10m radiation pattern:

Unfortunately on 10m the pattern starts to create lots of lobes with good low angle gain lobes into the East and West. However, the coverage for North and South is not that great. Lobes are high and not much gain in them.

6m radiation pattern:

Well 6m, what can I say. We can see quite a lot of narrow lobes albeit with good gain.

And yes, I can use the antenna on 6m and have made contacts into Western Australia (VK6), Northern Territory (VK8), Queensland (VK4), New South Wales (VK2) and Japan (JA) however, they have been far and few.

The antenna has been modeled with real ground (poor) to give me an indication of what I can expect at my QTH. However, it is not a true indication of the actual radiation pattern, there is a House behind the antenna (to the west) which will change the characteristics of the antenna performance especially into the westerly direction.

As with every antenna the impedance (Z) changes with height, soil conductivity and of course surroundings. This will dictate what type of Feedline and or BALUN one would/should/could use, there is no silver bullet. I do use an Antenna Analyser (VNA) to learn what the actual Antenna System (Antenna + Feedline) Z at my installation is, e.g. what my ATM would be presented with. This does help me to use what feedline and/or BALUN etc. I would use to configure the antenna-system to my requirements e.g. adjusting the antenna at a frequency of 7.2MHz to create easier matchable impedances for the additional Bands/Frequencies I would like to operate on.

According to my previous model of a Delta Loop used as a Horizontal Loop (LAZY-D) the Z was approx. 100 +j10Ω at resonance, which would mean that a 2:1 BALUN would be the right choice to get a good 50Ω match. 

Compared against the previous installation, this model tells me that I could except the Z to be > 200Ω which would indicate that a 4:1 BALUN would be needed to get a good 50Ω match. 

If you'd like a bit more information on Delta Loop antennas, a good educational read is the work of W4RNL (SK). His notes on Delta Loops can be found here. 

So to wrap it up, the block does not lend itself for the installation of a multiband dipole type of antenna (G5RV or ZS6BKW) so the next best thing for me was to grab something out of my box of tricks and go back to an old, but proven concept.

I didn't really wanted to add an VSWR prediction chart but due to multiple requests, here is a quick SWR plot of the modeled antenna:

Here are some SWR plots before the removal of the BALUN. And a view of how the antenna performs can be found here.

Profiling my antenna using WSPRNet

First things first, if you have a beam or more than one, on a mast or tower that nearly reaches the sky you might want to move on as this is more a process for us mere mortals that live in suburbia and have to live with a piece of wire.

 

I've been trying to get a "feel" for "how my current antenna is performing at my current QTH"! The antenna in question is a 40m Delta Loop, which is slopping from the roofline into the backyard. Now I can model the antenna with one of the many Antenna Modeling programs and using the results I can evaluate if the antenna will perform to my satisfaction. However, it is a model and it will show the antenna to work in an environment that is not indicative to the real environment.

So the next best thing I've done in the past use data from my logbook to get an understanding of the performance of my antenna. However, all this can be speed up these days in using WSPR. I'm not here to explain how WSPR works, except I will reuse the description given by the good folk at WSPRNet.

The Weak Signal Propagation Reporter Network is a group of amateur radio operators using K1JT's MEPT_JT digital mode to probe radio frequency propagation conditions using very low power (QRP/QRPp) transmissions. 

So basically I've setup a WSPR receiver over a period and reported the data to WSPRNet. I could also store the data in my own database, but why reinvent the wheel. Having all the received station data available we could now download all of the data and run it through a graphing/data analysing tool like grafana, gnuplot or Octave.

But why if we have a first class tool written by VK7JJ online directly getting the data from WSPRNet so I don't have to bother with anything else but look at WSPR ROCKS!

As I mentioned I setup a WSPR receiver to monitor the Amateur Radio WSPR frequencies from LF (136kHz) to end of HF (28MHz) and send the collected results to the WSPRNet. I then use the WSPR rocks website to filter out the Band of interest, use the biggest amount of data sets (limited at 5000), select unique calls and select an appropriate time frame.

Lets use the above selected dataset. Initially, the data will be displayed as text first. And to graph the dataset I select <charts> and then <SNR compass>. The result can be seen at the below graph. It is the result from all unique calls which my WSPR receiver heard over a  three (3) day period on the 20m WSPR frequency.

 

I can already see that my antenna is favoring only two directions, more than any other direction.

The question is, can I be sure?

It sure is a good start to visualise the real receive pattern but there is another dataset we can check and then compare it to my recorded dataset. That dataset is the <everyone> dataset. We can select that dataset for a comparison check by setting the <RX call> value (currently set to vk5hw) to everyone. And voila we get ...

This tells me that from 0° to 360°, i.e. around the globe stations where reported for the same time period. Comparing this graph with the graph from my dataset confirms my initial observation. It shows that I have two good and one sort of ok direction were the antenna is performing. The directions between 60° - 110° and 270° - 300° are pretty good. 315° - 25°, well I do receive signals but it seems to be a struggle. There are additional single signals around the 240° and 135° mark but they would be the exceptions due to ... maybe enhanced propagation wich I could find out with a propagation tool (Voacap or GWPS). Now there could have been por or no propagation between me and all those other stations. But even so, I'm confident that this is a good profile of my 20m receive capability at my QTH with the current (40m Delta Loop) antenna. 

There might be the ocasion opening that might give me a better path into those regions which I've received no signal from but those are more likely the exceptions and not the rule. 

Since antennas are a reciprocal passive device, meaning they work the same on receive (RX) and transmit (TX) I'll know which areas of the globe I'll be making easy contact to and were I would have a difficult time to work stations. 

So this is the profile for 20m on the 40m delta loop, and since I use the antenna for 40m, 20m, 15m and 10m as a TRX antenna I need to run profiles for at least 40m, 15m and 10m. Of course I'm also interested in the bands that I only use the antenna as a RX antenna so a few more graphs to compare. However, all I wanted to show is how easy and quick it is these days to profile, i.e. get a good feel, for ones antenna installation at ones QTH. 

Well, most of this is not new to me, but it confirms that I could have done this in three (3) days rather then in 24 month. That's how long it took me to to get to the same conclusion by operating FT8 and SSB from this QTH.

 

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.