Monthly Archives: July 2015

More measurments on the Magnetic Loop Antenna

Band conditions yesterday on 20m were really quite bad. I had difficulty even maintaining PSK31 contacts due to QSB. I asked W3OQ across town to monitor my 5W transmission on 20m using the Magnetic loop. He was able to pick me up without issue. He was coming in at about a 54 with 100W from across town so this was further confirmation that the conditions were limiting performance evaluation. The good news is that the antenna did radiate!

So now on to the tuning range issues. I was expecting to be able to tune this antenna between 20m and 10m. As it stands now I can do 20m and 17m only. This implies that my capacitance at the low end is higher than expected. I have actually taken the antenna apart now to do some modifications to the mast and used the opportunity to re-measure the capacitance range. I have gotten some inconsistent results. The lowest measurements I get are 9pF to 108pF. The highest measurements I have gotten are 15pF to 155pF. Using AA5TB’s Magnetic Loop Calculation Spreadsheet I get the following table of expected values:

Frequency – MHz       Capacitance – pF
7                                  174
14                                    43
18                                    26
21                                    19
24                                    14
28                                    10

Using the Mini60 Antenna analyzer with the cap at either extreme gave me a result of 8MHz with a predicted capacitance of 133pF and on the other end 20MHz corresponding to 21pF. This indicates that the actual capacitance of the system is running a bit higher than my measurements. This sets up the following experiments next time I get the antenna reassembled:

  1. Tune antenna to each frequency and then measure the capacitance at that point to compare against predicated values
  2. Determine how much capacitance needs to be added to achieve tuning on 40m. This is fairly straight forward by placing various fixed capacitors in parallel with the existing variable cap. My thought is to add a switch to allow use on 7m

The big problem will be fixing the tuning issue on the higher bands. I am searching for another capacitor now just in case. This will pretty much all wait until I receive the 6:1 shaft gear so that I can improve the tuning sensitivity. Meanwhile I am going to work on the tripod mount and the mast to get rid of the cable ties.

UPDATE: Found the actual specs on the capacitor I am using an confirmed that the range is 20 – 120 pF. This would confirm the tuning range from about 8.4MHz to 20.7Mhz. This is a two gang Capacitor with a range of 10 to 70pF and the other with a range of 10 – 50pF. I have these wired in parallel giving me the 20-120pF range. I should be able to use just one gang and get the range back in line. Simple enough fix!


Initial Measurements of the Magnetic Loop Antenna

Here is a plot of the Magnetic Loop Antenna tuned to 14.228 MHz in the 20m band.

loop20m Custom

SWR was 1.10 and the antenna was at resonance with a bandwidth of 40 kHz as predicted.Note that these measurements were taken indoors while mounted on a camera tripod just over three feed high.

This is all a good sign but there is also some issues that I will need to resolve.

First up is the tuning sensitivity. I have ordered a 6:1 reduction drive gear to help this issue. I just cannot fine tune adjust the capacitor to a specific frequency without a reduction mechanism. Next is the sensitivity to my hand being close to the capacitor while tuning. Ultimately I will solve this with a motorized tuner. For now I will just extend the tuning knob with a dowel stick.

Now for the bigger issue. I had originally designed this antenna for use with 20m through 10m limited largely by the range of capacitor I had available. i had settled on buying a piece of coax 9 feet long giving me a diameter of about 2.8 feet. I ended up buying a 10 foot length by mistake so the diameter is actually about 40 inches. While this should improve loop efficiency slightly it has messed up my plans on the high bands. Running a sweep at the extremes of the capacitor range give me a useable range of between 8Mhz and 20Mhz. The low end does not worry me much as I can add a cap in parallel to the variable cap to get the tuning in range. The bigger problem is with the high bands. Plus, I am not sure why they are not in tuning range as my capacitor goes down to 9pF which the design models show is OK.

Stay tuned (no pun intended) as this is a work in process…

Magnetic Loop Antenna

Powered through the construction of the magnetic loop antenna for HF last night after a last run to Home Depot for supplies. I had purchased three different capacitors from Ebay for this. One was unsuitable due to voltage handling. One was great for voltage handling but a bit bulky and stiff to turn. The one I decided to go with has a capacitance range of about 10pF to 110pF when both halves are ganged together. Voltage handling should work well with QRP and it is not too large. I found a 4x4x2 plastic junction box that housed the capacitor and the connections nicely.

I had spent some time thinking about how to motorize the cap from the start but I started having some issues with that and decided to try manual tuning to start out with as I can always add on the motor drive later on. Wiring up the cap in a clean way is by far the most difficult part of this project. I am using a 9 ft piece of RG-213 for the main loop so I added some SO-239 panel connectors to the box. This is handy as I can always replace the loop with different lengths of coax and try on optimize operation on other bands. Here are some photos of the capacitor build progression:

Photo Jul 27, 9 21 19 PMPhoto Jul 27, 9 21 44 PMPhoto Jul 27, 9 21 34 PM

Photo Jul 27, 9 46 54 PMPhoto Jul 27, 9 56 57 PMPhoto Jul 27, 9 59 49 PM

The coupling loop was made from an RG-8X patch cable. I cut off one end of the connector and formed a loop 1/5 the diameter of the main loop. The loop was wired as an unshielded type loop. The center counductor on the feedline end is soldered to the loop braid. On the other end the loop braids are soldered together. Once soldered I used electrical tape to cover the joints and give them some strength.

Next up was putting the whole thing together as I was anxious to test it out. The main frame is built from four pieces of 2 foot 3/4″ PVC pipe and a PVC cross. I had these on hand and the 3/4″ PVC fits on my camera tripod so I went with it. I may try and change this to 1/2″ PVC for the final version. I laid out the parts on the floor and used cable ties to hold the coax loop and the coupling loop to the frame. The box was a problem so I temporarily used electrical tape to tie it the PVC pipe. I’ll secure it long term with a clamp (back to Home Depot…), Here is the finished antenna ready for testing:

Photo Jul 27, 10 50 17 PM

Once all this was done it was about 11pm local time here at the QTH. I set it up in my Shack and connected it to my FT-817ND. I found a QSO and tuned the antenna for maximum signal strength on the S meter. Receive on this antenna is really pretty good. I was picking up signals that I could not hear well at all on the end fed wire and FT-450D. I picked up EA3AKP in Spain with a solid 59. Tuning up to transmit was a bit frustrating and I was not successful in getting the SWR down. The Q is so high on this antenna that without gear reduction it is very hard to fine tune the capacitor so the antenna resonates. Overall happy that it is “somewhat” finished and that I can start experimenting with it. Later today I will try again and use the Mini60 to try and tune it to resonance.

Email over HF

The SignaLink was the key to getting RMS Express and WINMOR to work last night. I sent a short email message to my gmail account and then received the reply. Transmission was on 40m to a gateway located in Houston.Transmission speed was around 230 bytes per minute. It is slow but it works and demonstrates that email can be exchanged globally without the need for expensive and dedicated packet hardware.

Installed a SignaLink for Digital modes

Installed a SignaLink USB for working digital modes to replace the digi-interface I had used previously.

Photo Jul 23, 6 37 56 PM

It works a little different than what I am used to in that the transmitter stays at 100% power and the power output is adjusted using the SignaLink’s TX control. I tested it out on PSK31 and made two contacts with no issues reported at 30W. The SignaLink also lets me use the Faros software for the first time. Faros monitors global beacon frequencies on HF and plots which stations are heard. The nice think about Faros is that is uses my antenna and receiver system at my QTH so the results are real-time view of current propagation conditions.

Adding legs to my FT-817ND

I really enjoy using my FT-817ND in the field but one issue I have is propping the radio up so i can read the display and work the dials. I came across this on Ebay and decided to try it out. This arrived fairly quickly from Hong Kong and was a snap to install…that is once I got the existing screws off. I stripped one of the screw heads and a simple job turned into a major hassle. I removed the screw by first covering the microphone and other exposed jacks with some electrical tape. Then using a small “micro” file cut a new slot into the head until the screwdriver could do its work again. Once this was off, installing the new legs was easy. Here is how they look deployed:


Thinking of a 2m Yagi

Looking at the Yagi again for 2m. The Moxon had a predicated gain of about 7.5dBi. This is roughly equivalent to a 2 element Yagi. So to get any improvement I would need to go with a 3 element Yagi. I’ve layed one out and here are the EZNEC predictions:


The compromise here is reduced beam width vs. higher gain. This would give me 12.1dBi of gain vs. the 7.5dBi on the Moxon.My guess is that at 5w I will be heard simplex in Weslaco and improve reception from the upper valley. I will probably put this on hold as I now have the parts I need to start my HF magnetic loop antenna project. I am also hoping to get an FT-857D soon that would give me 50W of VHF capability.

SImplex Testing of the 2m Moxon

Setup the 2m Moxon on a painters pole late afternoon yesterday and checked into the local daily ragchewers net. Was able to hear the repeater traffic in Brownsville, Harlingen and La Feria at full quieting. South Padre Island was readable but scratchy. Could not hear the Edinburg or Raymonsville repeaters at all. W3OQ in Olimto helped me do some testing. I was able to work the La Feria repeater with no problems with full quieting reported on the local repeater node.

Photo Jul 21, 7 24 24 PMPhoto Jul 21, 7 24 56 PM

On simplex I heard stations from Mcallen/Mission and Weslaco for the first time. The Weslaco station could hear me only once. Stations in the upper valley were fading in and out but could not hear me. W3OE was reporting hearing stations near Corpus Christi and received a text message that he had been hear near Refugio.

So to summarize, the 2m Moxon does show an improvement as predicted by EZNEC. At 5W this is not enough to be heard widely through region on simplex but does give me access to a number of other local repeaters. Note that many of the simplex stations I was trying to work are on 50W and have antennas mounted up around 40ft.

Basis for my 2m Moxon

The basis for the 2m Moxon I built is here (2m-moxon-antenna). This describes the basic approach to building the 2m Moxon as developed in 2008 by the Montgomery County ARES.  I did make some modifications to this in order to deal with the parts I got from Home Depot. The end caps and PVC tees have one length but the PVC crosses are a bit shorter. I used a caliper to measure the length of the Tee sub-assembly then set the crosses to match. I then calculated how much wire needs to be inserted in order to set the right gap between elements which in this design is critical. I used a smaller drill size to make the holes so that the wire would be a fairly tight fit. Then I wrapped electrical tape around the wire to form a “stop” so that the wire would not go in further than necessary.

I cut the connector off of one end of a RG-8X coax cable and fed it up the PVC pipe to the feed point. In retrospect this was a mistake as now I cannot easily add a mounting extension to the cross. I will compensate for this by slitting a short piece of PVC pipe to allow the coax to emerge. I have a slip – thread coupler that will fit on the end of this so I can get the antenna mounted o the painters pole for testing. I am using a bulkhead adapter to connect the two pieces of coax for the feed line and have a small patch cable that connects to the Baofeng HT. Will be ready to test later today at new heights.

Finished the 2m Moxon Today

I finished up the 2m Moxon antenna today. I had bent the copper wire elements a few days ago after having straightened the #8 solid wire. Today just had to make some measurements, cut some PVC pipe, drill a few holes and solder up the feed line. All told a pretty quick build with the main issue taking time to get the measurements right. I called up W3OQ across town for a simplex radio check with the Moxon hooked up to my 5W Baofeng HT. I held it up, oriented vertically as high as I could reach and W3OQ reported full scale (13 bars) at his QTH. My 4W HT with the discone up about 12 feet scored 9 bars. Confirmed that there was directivity. Signal dropped to 5 bars on the backside. Looks like for an initial test it worked fine.

I am very impressed with how compact this antenna is. It is a rectangle only 29 inches by just less than 11 inches. Next steps will be to try simplex again with the antenna mounted up as high as I can get it on the painters pole. Here’s a picture of the completed Moxon

Photo Jul 16, 10 36 58 PM