Tag Archives: eznec

Musings on Vertical Beam Antennas

I have been doing some EZNEC & Propagation studies on various vertical configurations. First off is the Ground Beam QEC antenna. The EZNEC model looks like this:

gbqec

The model indicates (and the performance seems to confirm) that this antenna is directive but the small number of ground radials causes gain to suffer. The main benefit of this antenna over a vertical is that it has some F/B reduction. Running some optimization on this models indicates some improvement by significantly lengthening the reflector. The gain increases slightly but the F/B ratio improves. Unfortunately, the reflector would have to be 7m long for a 20M antenna making deployment difficult using the telescopic fishing poles.

I was thinking about the homebrew Buddistick I built a while back and remembered it exhibited modest forward gain in the direction of the single elevated radial. Here is the model:

buddistick

This antenna is somewhat directional but also has considerable high angle radiation. It would still appear to be a better performer as a beam than either the 1/4 wave vertical or the ground beam QEC antenna. Drawbacks?…not as lightweight.

My final candidate is a 2 element vertical beam. Making a few compromises in mounting I can use the fishing poles to make a full size 1/4 wave vertical on 20m using a single elevated radial. The feed point is only 3 feet off the ground. Here is the model:

2el vert

This can be built in a manner very similar to the ground beam QEC. I just need to figure out how to mount the ends a bit higher. Horizontal space needed is about 16 feet greater than the ground beam QEC. here are the radiation patterns from the model:

pattern

To summarize this chart, the GB-QEC is almost equal to the 1/4 wave vertical in forward direction but with some rear rejection. The 20m Buddistick has greater forward gain, some rear rejection but considerable high angle radiation. This antenna makes a great Field day antenna at my QTH as it can point north and cover most of the USA and Canada. Finally the two element vertical beam provides 3db greater gain then the 1/4 wave vertical with 8db F/B ratio and good low angle radiation.

This info leads me to want to try the 2 element beam next. These parasitic beams are of course fixed in direction. The phased 2 element vertical array has the advantage it will provide some steering.

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Practical Application of Soil Measurement Data

Anyone interested in ground radials, ground mounted verticals and even elevated radial verticals should visit Larry Severns, N6LF, website. he has done extenstive research om the subject and has published his results in QST and QEX with many of these articles available on the website. His article ” Experimental Determination of Ground System Performance for HF Verticals Part 4 How Many Radials Does My Vertical Really Need?” was originally published in QEX May/June 2009 edition. The following graph was taked from this paper:

2017-10-09_11-40-24

This graph shows the change of signal improvement on a 1/4 wave vertical with 1/4 wave ground radials under different ground conditions. The impact of this graph is that when the ground is “good”, adding more radials results in small signal improvement vs when using a “poor” ground. I can expect a .2dB improvement if I double my radials from 8 to 16. If I double again to 32 radials my signal improvement would be only around .5dB.

There is a real economic benefit to having good soil as in general fewer radials are needed because the ground losses are less than when over poor soil.

More on Soil Meaurements

It is extremely easy to take soil electrical data using the RigExpert by running a frequency sweep. This sweep is saved on the meter and then transferred to the PC and exported as an CSV file. This file contains impedance measurements across the sweep range. Add the formula to convert to conductivity and Er and you get the following graph:

2017-10-09_10-03-40

This data was calculated with a Co = 8 pF adjusted after making a few more measurements. The average values per band are as follows:

2017-10-09_10-15-21

The values above can be compared to those given as standard values in EZNEC. I further compared the gain results when using a 40m 1/4 Wave vertical with four ground mounted radials as follows:

2017-10-09_10-26-56

In this case, the model has a 2.27dB difference in measured gain between the average soil case and the measured reading. In practice, using the “Very Good” values will likely be close enough for modeling work here at the QTH.

 

A 20m Vertical Beam

JP1QEC developed a portable 20m vertical beam he calls the “Garden Beam Antenna“. I built one this afternoon and have it pointed towards Europe. Construction is stratightfoward. I used a fishing pole to first make a 20m 1/4 wave vertical. It has two ground radials that are about 18 feet long. A third radial extends 18.7 feet from the driven element in the direction opposite where you want the main energy to go. This is connected to two more 18 ft radials and the vertical reflector element also mounted on a fishing pole. I ended up with a very flat SWR of 1.6 across the entire 20m band. I suspect that there needs to be some tweaking to optimize the pattern but I am seeing some directionality based on signal reports from making some JT-65 contacts. The beam is pointed on a bearing of about 40°. I get strong reports from stations in the northeast while stations on the west coast are copying my signals considerably weaker. I made several SSB contacts with most reporting 59+. I’ll be testing the configuration in some more depth over the next few days and comparing with EZNEC results.

Holiday Project Update

Managed to finish rewiring some of the shack for shorter cable runs and lower RFI. The shorter cables are especially handy on the FT-817ND. I can now neatly accommodate any combination of radio, amp, tuner and SWR meter. It is crazy windy out here today so I will likely wait until next week and setup the Buddistick and run the FT-817ND at 45 watts portable.

The APRS cable project has been de;ayed by the lack of a correct cable for the radio. The correct cable should arrive on Tuesday so I will wrap that project up then.

Been rethinking the 20m-10m portable beam project and am doing some EZNEC modeling. The issues I am looking at revolve around a portable 20m 2 element yagi that I can only raise up to about 20ft versus vertical buddisticks.

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:

2015-07-22_12-25-582015-07-22_12-25-43

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.

More on a directional VHF antenna

Now thinking that a better choice over a Yagi for 2m is a Moxon. The Moxon has a few really positive attributes:

  • Reduced size – folded elements
  • 2 elements have approximately the same gain as a 3 element Yagi
  • Large Beam width and Front to Back ratio
  • Easy and cheap to build

For 2m the Moxon rectangle works out to about 29″x11″. Here are some EZNEC predications for such an antenna realized with #8 copper wire:

2015-06-30_12-52-332015-06-30_12-52-55

I am going to try an build this by making a frame out of 1/2″ PVC and then putting the #8 wire around it and holding it in place with zip ties. This should be a fun and inexpensive experiment for 2m

VHF Yagi Ideas for Simplex Work

Some local hams have been experimenting with VHF simplex for communicating up and down the valley. The guy who usually leads these experiments is located in Brownsville and has his VHF vertical mounted on a 40 foot tower. He is able to communicate with other hams up and down the valley depending on the other hams configuration. I can communicate easily with him from my home QTH but I am deaf to all but one other local ham on simplex.

My VHF station is spartan at best. I use a 4W Baofeng handy talkie connected to a discone antenna that is up about 10 feet. I get no gain from the discone. This setup works real well for hitting the local repeater and for limited local simplex. I started thinking about the geographic location in respect to other stations in the valley and plotted the following map:

2015-06-30_7-16-07

My QTH in Brownsville is in the lower right hand side of the map. The Rio Grande River is the US-Mexico border and the Gulf of Mexico is to the East. To cover the valley I only need a Beam width of about 126°. I started thinking about building a 2 element yagi vertically oriented and came up with the following azimuth pattern using EZNEC:

2015-06-30_8-51-10

This antenna would give me a 133° beam width and a gain of 7.5dB over what I have now. My only concern is the rather low take off angle of the main lobe. Clearly the higher the better for this antenna but it looks like it would be a good starting point for experiments. Should be fairly simple to build from PVC and aluminum tubing. Will post more as I progress on actually building this.

Portable Antenna Analysis Roundup

Portable Antenna Analysis

The link above is to a file containing a EZNEC analysis of various antenna types I am looking at for portable use. These are mainly based on the Homebrew Buddipole and Buddistick designs that I have been working on with an emphasis of operations between 20m and 10m. Right now I have a decent setup with the vertical Buddistick in this range. It is easy to deploy on either band. The homebrew Buddistick has worked great on 10m. The jury is still out on 20m as I have not tested it much yet. The next iteration is a homebrew Buddipole dipole antenna. The EZNEC studies show better 10m performance if the dipole is at least 16ft above ground. Not much difference between the vertical on 20m but the 20m dipole needs additional height which starts getting tricky. The nice thing is that I should be able to use the dipole as the driven element for a two element Yagi on a short boom. The simulation shows good directivity and significant gain increase on 10m at 16 feet. Once again the 20m version requires more height to be effective vs. the vertical.

This analysis is still a work in process but this is showing the direction I plan to take the antenna work. I will use the Vertical Buddistick as my main antenna system for portable use as it is already built and is easy to deploy. Next step is complete the 10m Buddipole dipole. This will work thru 15m as well by extending the whips. I will go ahead and wind the coils for 20m operation even though it looks like at 20m the antenna will be extremely compromised. Once I have played with the dipoles I can use them as the driven elements for a 2 element Yagi. This should provide the best performance on 10m thru 15m. Stay tuned and I will post pictures and maybe some video on my progress.