MFJ Octopus

My First Impression

In the deep dive into searching for the perfect portable antenna to take on camping trip that is also suitable for NVIS propagation one that came to mind is the Multiband Ham Stick Dipole setup.  Upon receiving the MFJ-2104 with the included 15, 20, 40 and 75 meter Ham Sticks i was not surprised to see minor damage to the unit itself, as MFJ has become known over the past several years for not having the best quality control of their products.  This is not to take away that they do have some decent products, just that minor issues are now expected.  The issue with the one I received was that the set screw insert for attaching it to the mast was pushed completely through the sleeve, easy enough fix to not warrant an exchange.  Other than that the MFJ-2104 seems to be a decently built design that should last for a while with minimal maintenance.

Initial Set Up

Prior to the initial set up I went through each of the Ham Sticks and attached the endcaps with set screws for the elements making sure they were all tightened properly. Prior to inserting the elements I went through to mark each at the recommended length (4″) from the internal end for a starting point in tunning the elements.  After this I took everything outside and attached the Octopus MFJ-2104 to my portable fiberglass mast and then began attaching each of the Ham Sticks making sure to put the 40M and 80M elements at a 90 degree offset each other, same as you do with a cross dipole setup.

Now the fun began with tunning the different bands and seeing how much of an effect they had on each other in the tuning process, surprisingly not nearly as much as I thought it would. 

After another round of adjustments, another one inch on 15 and 20 meter and 2 more inches on 40 and 80 meter I found the SWR getting real close on all the bands so another round of adjusting was in order.  The 15 and 20 meter only needed a quarter inch additional to bring the SWR down to 1.5: and 1.6:1, while not perfect it was acceptable for the initail round of testing.  On the 40 meter and 80 meter elements I went another inch on adjusting which got the 40 meter to an acceptable 1.5:1 but the 80 meter only came down to 2.3:1, so another round of adjusting was needed.

Leaving the 15, 20, and 40 meter alone I adjusted the 80 meter another half inch and re-tested.  This brought the SWR on 80 meter down to 1.3:1 right at 3.815 MHz which was great considering I had an 80 meter NVIS net the following morning.  However this also changed the SWR on the other 3 bands, 40 meter was now 1.9:1 with 15 and 20 meter both moving to a 2.0:1 SWR.  While still acceptable I wanted to try and get them a little lower so I adjusted those three bands 1/8 inch and got them all below 1.5:1 but it did move the low point of the 75/80 meter element to 3.825 MHz, not that big of a deal since it was still close to the needed frequency.

Bandwidth

As expected with any loaded antenna the bandwidth is determined by the length of the loading coil.  What I found is that on the higher HF bands, 15 and 20 meter it is possible to tune the elements that will allow the entire amateur band to be used.  It may require the use of a tuner as you near the edges of the band, but this can easily be accomplished with any internal tuner if the radio has it.  On the 20 meter I tuned the elements right at 14.175 MHz  and the entire band was at or below 2:1 SWR, the image below for 20 meter shows a sweep from 14.100 MHz  to 14.350 MHz which includes all of the phone portion of the band.  At 14.225 MHz the SWR was at 1.46:1, which is great for a severely compromised antenna in my opinion.  The 15 meter elements showed the same with tunning at 21.200 MHz the entire band from 21.000 MHz – 21.450 MHZ showed well below 2:1 SWR, I’d expect the same from the 10 meter Ham Sticks when I get them.

However the lower HF Bands such as 40 meter and the 75/80 meter elements I tested show a very narrow bandwidth of usable frequency range.  The picture of the NanoVNA screen for 40 meter below was while tuning was in progress.  As you can see from the sweep of 6.750 MHz – 7.150 MHz (attempting to find the dip) it shows an SWR of 1.33:1 at 6.995 MHz with a narrow dip.  The SWR at 7.150 MHz shown was near 5:1, while it could be brought down with an external tuner in most cases, it would not be remotely efficient in compromised antenna standards. On 40 meter I was able to get the dip right at 7.200 MHZ with an SWR of 1.3:1 with a usable bandwidth from 7.100 MHz – 7.280 MHz with the SWR staying below 3:1.  Outside of that range the SWR drastically increased with 7.074 MHZ being nearly 6:1 and 7.970 being nearly 5:1.

Now for the REALLY narrow bandwidth of the 75/80 meter elements, I did not get any screen captures of this but will add some in the future.  In order to use this band it is extremely important to know where you will be operating as you will be dealing with a very small sliver of usable frequency range in a fairly wide area of use.  I was able to get the elements tuned with the dip at 3.825 MHz as I was needing 3.810 MHz for an NVIS net the following morning.  right at 3.825 MHz the SWR was below 1.3:1, while at 3.810 MHz the SWR was already up to 1.6:1.  The usable frequency range started at 3.780 MHZ with an SWR of 3:1 and went though 3.860 MHZ where the SWR was at 3:1.  Yes you could squeeze a little more out of the bandwidth with the use of an external tuner, but in my opinion it is not worth it due to losses in radiating power, and the heat created in the equipment itself.  With this information I would just know where you want to operate and plan accordingly.

First Use and Contacts

After getting the antenna set up I had to see how well it performed, and that it did surprisingly well for a severely compromised antenna.  I connected the antenna into my external tuner to use it as a switch so I could easily test receive and transmit with my already tested End-fed 6-80 multiband antenna.  I was pleasantly surprised to hear the noise floor on the MFJ Octopus was 2-4 dB less than the wire antenna.  I instantly heard a lot of traffic on 15 meter so I tuned on a pile up (the station calling CQ was in W. Virginia, I am located in Northern Colorado) and switched between the Octopus and wire to check the receive capabilities.  While the received signal was nearly identical the lower noise floor of the octopus made it nicer to listen too.  This showed promise so I decided to try and get through the pile up, after calling back 3 times I was through the pile up and had a complete QSO with a signal report of 56, on par for other contacts in that region with the wire antenna.

After making some more contacts in the continental U.S. I heard someone calling CQ from Hawaii (Pearl Harbor) on 21.375 MHz and figured why not try. This lead to a successful QSO with KH6KW with a signal report of 53, which was great in my opinion for only running 75 watts through a compromised antenna from the front range of Colorado. After this contact I switched to 20 meter where the luck continued and had many DX contacts ranging from as close as Canada, Mexico, Puerto Rico and as far away as South Africa and Finland, all on 20 meter.

The next morning I got on for the planned 80 meter NVIS Net and was pleasantly surprised by the noise floor being below S1.  I turned the radio on at 06:00 and did some listening and a couple QSO’s in the narrow sliver of the band I could use only to have the band conditions get really bad at 07:00.  After sitting around doing some other things waiting for the net to start up at 08:00, I was surprised to have a good copy on the NCS located 50 miles away with hills and mountains in between so I knew it could only be NVIS propagation. With 27 people checked into the Net (all around Colorado) i was able to copy 25 of them with a readability of 3 or higher, in most cases a 5, the two I did not copy were due to doubling when I checked into the net.  The benefit of this NVIS Net is it is run by our State ARES group and incorporates a Can You Hear Me Now (CYHMN) exercise with between 8-12 stations recording who they can hear with a readability report, so I will get some good data on the NVIS use of the Octopus antenna.  This information will be added when I receive it in a week or so, so check back if you are interested in it.

Over all I am pleasantly surprised with the overall performance, even given the narrow bandwidth of the lower HF bands.  I will continue to test and use this antenna and it may just become my main antenna when out boondocking in the mountains.

Update

After using this antenna as my primary base antenna for nearly 2 weeks I am pleased with its overall performance.  While the lower bands (40, 60 and 80 meter) are very narrow banded with proper planning and tuning they are completely usable.  In regards to the Colorado ARES Can You Hear Me Now project I was successful in hearing over 90% of the stations checking into the net and was heard by more than 80% of the recording stations around the state of Colorado.  

In addition to the testing with the ARES group for NVIS propagation of the MFJ Octopus, I also pushed its limits for Coast to Coast coverage as well as DX contacts.  On the 20 meter side of it I was successful in making contacts to Hawaii, New Zealand, Australia, Japan and Poland during the last couple days of use.  This is incredible for a compromised dipole antenna made from two ham sticks.  This will definitely be an antenna that gets packed when heading out for a day of POTA or a boondocking trip with the camper.

73, DE KA6ETE