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Covid-19 Dual Band 2m/70cm Antenna Build

Created by Mike Sanchez on 2020-06-24

A Covid-19 Dual Band 2m/70cm Antenna for $9

5.37 dBi (2m) and 7.39 dBi (70cm) at 14 ft

The Origins

In 2012, as I re-entered Amateur Radio, I dug out my old Yaesu FT-5100 that covered 2m/70cm bands for FM.   Then, at the Rochester Hamfest (at RIT in 2013 I think), I ran across an old Radio Shack scanner antenna (Model 20-176, thanks to W3OAB for identifying it after I bought it).   The seller claimed it covered 2m and 70cm and he wanted $5 for it.   I bought it, placed it atop of a 10ft, 1inch, schedule 40 plastic conduit pipe on my deck (14’ off the ground) and I could hit many of the local 2m repeaters but not much on 70cm.   The antenna was an interesting design with a shorter, rotatable V from the center where a longer radiator for 2m extended vertically.   You can see some reviews and a bit of what the antenna looked like at this link on eHam.   

https://www.eham.net/reviews/view-product?id=8026

                The Radio Shack antenna is also not really weather sealed, due to a design which enables rotating the V with plunger sealed by an O-ring, so it is possible for water to get down into the coax connector past the o-ring from the seam around which the V rotates.

Such was the state of my VHF/UHF antenna capability when Chris Shalvoy, K2CS, recently,  to keep RDXA moving along during Covid-19 lockdown, led us all to get together on WB2KAO’s 70cm repeater.   I had to scramble to come up with a way to hit his repeater, which, turned out, my handheld could do upstairs.  However, at the end of the nice remote meeting, I decided to find a good (enough for me) 2m/70cm antenna.

The Search for a Dual Band 2m/70cm antenna.

                First, I went to the dxengineering site and searched for a dual band antenna, which, promptly returned an 8’ fiberglass beast that I know my wife is not going to be happy with on the back deck and also, it costs $60 bucks.   Claimed gain was 8 dBi but it is impossible to have a single gain value for two bands so I decided to take a pass and save the $60.

                Second, I went to eBay where a bunch of big, copper J poles are sold for around $30-$40.  Again, I did not want such a big antenna, and, my 10 foot conduit won’t stand up under the weight, and, the 70cm J pole is not exactly a great radiator.

                Third, I searched for build your own instructions for dual banders  on youtube and there are some great videos out there, one of which I thought might have some promise.  It was a 2m Quarter Wave ground plane, built off of an SO-239, with two little V type 70cm radiating elements also attached to the center SO-239 connector; similar to the old Radio Shack dual band. 

4NEC2 Simulation exploration – “Radio Shack” type of dual band

                To better understand the type of antenna with two radiating elements with 70cm in a V, I built a simulation of dual bander with two V type radiators for 70cm and one vertical radiator for 2m.   From simulation, the antenna is OK for 2m, however, it has a very directional radiating pattern for 70cm with peak gain nodes in the direction of each of the radiators and fairly deep nulls on the side.   I now understand why the Radio Shack antenna enabled rotating the V section (in spite of it being on top of a mast and not accessible).  It is likely the nulls helped create the impression of a bad antenna.  Overall, I wanted something omni directional, wtih better weather sealing, on both bands.

What to do?  A 2m/70cm Compromise

                At this point I thought maybe I would look through the 4NEC2 examples that unzip with the install with 4NEC2.   I found a file, under the directory VHFsimple, with the name:

“2mGP.nec”.   

When I opened the file, in the comments at the top, I found:

CM   2M Quarter-wave Ground-plane

CM  (2m/440 compromise)

The antenna looked like a fairly simple file, and, it appeared to cover both 2m and 70cm.  Unfortunately, the antenna model in the above file is constructed and optimized in free space, not relative to ground and not with an obvious selection for wire diameter.  

However, I did convert the file to have a real ground, and, explored a variety of lengths to understand, then, design an optimal outcome for both 145 MHz and for 445 MHz frequenices.   The antenna arrived at is shown below, and looks very similar to a standard 2m quarter wave ground plane except radial and radiator lengths and radial angle.   However, the dimensions are sub optimal for min SWR for 145MHz but enable a 3/2 wavelength antenna on 70cm as we will see. 

Figure 1:  “Compromise 2m/70cm Dual Band Antenna” 

Optimal Compromise For 2m/70cm @145.0 MHz and 445.0 MHz

                To compress a couple of days of exploration in simulation into a short summay:   I began by exploring the three variables radiator length, radial length and acute angle.   However, to constrain the optimimzation and the ultimate real build, I ended up setting the length of the radial and the vertical radiator equal and varying that single variable with the radial acute angle with vertical.

                After too much exploration because, mostly, of ignorance on my part, I finally arrived at dimensions that did, indeed, give me an antenna that is acceptable, in simulation, for SWR, on 2m and 70cm.  

Finalized dimensions of the system are, from the model:

Height above ground:  14 ft

Length of vertical radiator and Radials = 19.3 inches (simulation, not experimental).

Distance of bottom of radial from a vertical line from the center of the vertical radiator = 11.0 inches.

Length of radials 19.4 inch. 

4NEC2 Simulation, Optimal, SWR Results for 2m/70cm Compromise. 

                The optimum for the “compromise” antenna mandates that a minimum SWR occur, between the two bands, at both 145 MHz and 445 MHz simultaneously.   Practically speaking, this ends up being a crossover point at 145 MHz in SWR since, the optimal for 145 MHz will be a bit too long and the optimal for 445 MHz will be a little short in length. 

The simulation SWR results for both bands are below (with a little help from Python):

Figure 2: 4NEC2 Simulation SWR  results for single radiator, dual band, 2m/7cm antenna

As can be seen in Figure 3, there is a minimum SWR crossover point at around 145/445 MHz.  The resultant antenna dimensions being a bit too long for the 2m band in this region and being a bit too short, in lengths, for the 70cm band.   However, in simulation, most of each band has an SWR of below 2.0.   For 440 MHz, given the somewhat signficant coax losses already with even perfect 50 ohm radiators, maybe this would not be acceptable for contesters in the 444-442MHz range, but, for me, where I just want an effective antenna for local repeaters, and, the one in Ontario across the lake, it seemed like an attractive prospect. 

Simulation Far Field Pattern 2m/70cm “Compromise” - 5.37 and 7.39 dBi

For the potential compromise antenna, with SWR results shown in Figure 3, the far field patterns are good for 2m, and, very good for 70cm.   Because the radiator is 1.5X the wavelength of the 70cm band, the effect of that longer length results in excellent low angle, far field radiation as shown below.  

Figure 3: Dual Band, 2m/70cm  Vertical Radiation Pattern

From the above simulation results the 2m max gain occurs at a radiation angle from horizontal of 6o at 5.37 dBi and the 70cm max gain occurs at the very small radiation angle of 2o .   These are excellent gain numbers, and angles, espeically for 70cm.  

Since the gain looks good, and, the SWR looks acceptable, I decided to build this antenna in a robust, lightweight, framework – and test it. 

Building the 2m/70cm Compromise Dual Bander.

                I had found in both text and on youtube, single band, quarter wave antenna’s for, especially, UHF built on, and around, an SO-239 itself.  

One great site is: https://m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/

So, I decided to build the antenna around the SO-239 platform, then, weather seal the resultant antenna with epoxy where the vertical radiator emits from the insulator.   First, I searched on line to find brass rod of various diameters.   After some searching I found brass rod in 1/8 inch and 0.09375 inch diameter in six foot lengths for $1.85 at the link below.  Shipping is not cheap but not too expensive if you order a fair number of the very, very cheap brass rods at $1.85 per 6 foot length.  

Radials

Figure 4:  Brass Rod source (6’ of 0.09375” rod for $1.85). 

https://www.onlinemetals.com/en/buy/brass/0-09375-brass-round-bar-360-h02-extruded/pid/7357?gclid=Cj0KCQjwhtT1BRCiARIsAGlY51K9XeSgKmfVMb78uePQc4CJJN1AMQM1A0rZVIoeSBV5-wazaU3Twl8aAvawEALw_wcB

                Once I received the rods I experimented with copper based electrical ring terminals and found that wire size 14-16 “just” allows the 0.09375 inch rod in (with a little help).  It is a very tight fit, but, the 14-16 ring terminal  fits and that is exactly what I wanted.   I already had an SO-239 from dx engineering and utilized that, with solder, to attach the brass rod radiator.  Ring terminals were sourced from Lowes.

                I cut each length equal since simulation optimal was very close to equal and cut them longer than than the 19.3 inches of simulation, 19.75 inches, to ensure I did not have a too short length from the start.   Below is a photo of each length of brass rod for the radials, along with the brass rod attached to the SO-239 via solder. 

Figure 5:  Brass Rods prepped with Ring Terminals and SO-239 soldered to brass rod.

Figure 6: The fit of the 0.09375” brass rod into 14-16 ring terminal wire terminal.

Figure 7: Ends were soldered to the terminals at this end (with old, leaded solder).

I had been concerned about having the plastic on the radials, but, when I soldered the ring terminal and the brass rod, the plastic slipped right off leaving the uninsulated ring terminal.    

On a first iteration of this antenna I soldered the radials to the SO-239 base but that overheats the center insulator as part of getting the SO-239 to temperature.  So, on a second iteration, I used stainless steel machine #6 machine screws which fit very well and, together with a #6 flat washer and #6 lock washer are secure.

Figure 8:  Attachment hardware for fitting the Ring Terminals to the base of the SO-239

Radiator

I soldered one brass rod to the SO-239 open end for connection as shown in Figure 10.  As far as I could tell I avoided overheating the center insulator and avoided a cold solder joint.

Figure 9:  0.09375” Brass Rod Soldered to DX-engineering SO-239

Attaching the Radials to the SO-239

                To attach the radials to the SO-239 securely, I attached the non-bent (yet) ring terminals, upside down, to the SO-239 with the machine screw with the panhead side using the flast washer and the nut side using the lock washer (as was the common attachment approach on our farm years ago).   I lined up the radials so they are easy to attache while the SO-239 lays flat on a table.

Figure 10:  Radials/Ring Terminals Attached to SO-239 – before bending. into position. 

Why install the ring terminals “upside down” on the top of the SO-239?

                The ring terminals are copper, not steel, to promote good conductivty at high frequency.  So, they bend easily.   To insure that the radials bend to the same angle of the simulation AND stay that way, I installed the ring terminals upside down.  If you look closely at Figure 11, you will see that, when the radials bend downward the ring terminal will “stop” at the nut on the underside of the SO-239 at almost the exactly correct angle of incidence from the vertical plane in the simulation.  Because of gravity, the radials will stay that way indefinitely even with the soft, copper ring terminals.

Rotate, keep horizontal, and tighten nuts.

                The radials are now rotated into position, still horizontal, to enable access to the nuts on the underside of the SO-239 for tightening.   Radials are now in the proper position to tighten the nuts with a screwdriver on the pan head screw and a small wrench fit to the #6 nut.   These are small, must be tight, so, careful to avoid slipping the screwdriver on the panhead screw.

Figure 11: Positioning, tightening nuts and screws the assembly. 

Bending Radials Into Position

                The antenna is now lifted and the radials bent downward until they touch the small nuts underneath the SO-239, anchoring them into position.   In the picture below note how the top of the ring terminal, now inverted, hits the nut which acts as an angle stop.   The resultant angle is nearly perfect match to the simulation. 

Figure 12: Radials Bent Into Position, Touching the Nuts Beneath the SO-239.

Epoxy Sealing the Top of the SO-239 from the Weather.

                Once the screws and nuts are tightened, and, each radial bent to touch the nuts as shown above, then, I tested a bit to make sure that the assembly is tight and no radial moves around.  In this test I don’t yank on anything harder than a bit of wind might shake.   All of the assembly was tight and strong. 

Weather Sealing Effects of Epoxy on SWR. 

                I planned to weather seal with JB Weld Epoxy on the top of the SO-239 in the region from above the radiator solder point to the edge of the center insulator thereby covering everything that might have a seam for water encroachment.

                I read a as much as I could about the effect of Epoxy on high frequency use, but, there is not a lot out there, so, BEFORE I added epoxy I mounted the not Epoxied antenna, and, ran a full test of SWR on both bands.    To summarize;  the before Epoxy and after Epoxy results were only very slightly different but the measurements were also on different days so the ground might have been slightly different.   At any rate, I concluded Epoxy was OK for use 140MHz through 450 MHz.  To limit ths size of this report I will only report post epoxy SWR measurements on the antenna.

Sealing with JB Weld Epoxy

                JB Weld Epoxy was on my work bench so I used it.  I don’t remember ever buying it and, about the time I needed to Epoxy I glanced up from the bench and there it was.   Sometimes, luck actually does come my way.  I mixed a bit according to directions, then, used a toothpick to very, very carefully coat the surface of the radial shown in Fig 14 just above SO-239 to SO-239 center insulator and then outward from the radiator to the edge of the SO-239 center insulator.   Then, let it all dry for 24 hours before further movement of the antenna.

Figure 13:  Radiator and Center Insulator JB Weld Epoxy Sealed. 

Completing Outdoor Install of Antenna for SWR and Repeater Testing.

                I had previously acquired a 10’ length of 1” Schedule 40, plastic conduit pipe and erected it on my four foot off the ground back deck resulting in an antenna base height of 14’.  I now routed the coax up through the center of that conduit pipe and through the other end and attached the SO-239 on the bottom of the antenna and pushed it up into position as shown below in Figure 15.  Note, I used a cutout, inverted, clay potted plant holder with the bottom center drilled out in the center and placed teh bottom of the conduit on that for coax center routing easily up through the pipe.  

Figure 14:   Compromise 2m/70cm Antenna Up in the Air for Testing

Testing the Coax Cable First, then, Testing two lengths:19.75” and 19.50”

                I decided, before testing the SWR response at the radio end of the coax, to test the coax itself.  I installed a 55’ length of JSC Wire #3020 RG8/U in 2012, so, it is getting on the older side.  On the plus side, Rochester has almost no sun, so, sun damage of the outer coax is limited. 

I used my large, 50 ohm dummy load acquired from John Cunliffe, which for VHF and UHF is amazingly still exactly 50 ohms.   At the end of my Coax I placed a Bird Wattmeter with a 100-250MHz 5W slug then routed into the dummy load.   Then, I transmitted 2 Watts into the coax at the radio end and measured the Watts emitting from the coax:  1.5 Watt.

The dB loss of 55 feet for this older coax is:   1.25 dB, making the loss per 100’ = to 2.5 dB (with two connectors).   Comparing this spec to various RG/8U losses posted in various charts, I observed between 1.7 and 2.4 dB loss in this type of cable at 150 MHz or so.   I concluded the coax is OK.   

                For data analysis:  I used Python and wrote a generalized comparison class that enables me to feed two data sets and generate a graph with proper legend and easy data differentiation by color.   Below we can see that, for both lengths, 19.75” and 19.50”, the SWR is good, with 19.5 offer an slightly lower SWR across the 2m Ham band and 19.75 offers lower SWR on 70cm band. 

Figure 15: SWR Results Comparison 140-150MHz

Figure 16:  SWR Results Comparison 440-450MHz

Comparison:  19.5” Antenna Measured SWR vs 4NEC Simulation SWR

Figure 17:  2m Measured SWR vs Simulation SWR for Optimal Compromise

Figure 18: 70cm Measured SWR vs Simulation SWR for Optimal Compromise

In a general sense, the simulation and the actual antenna response are similar on both bands except the lower portion of 70cm which shows anomolous behavior and interaction with something TBD. 

Partial list of Repeaters Hit Using Compromise 2m/70cm

2m Repeaters                                                                                    70cm Repeaters

145.110 Bristol                                                                                   442.075    Rochester

145.150 Cobourg/Rice Lake ONT                                                442.800   Rochester

145.210 Churchville                                                                         444.250   Rochester

145.270  Albion                                                                                  444.350    Rochester

145.290  Canadice                                                                            444.400    Rochester

145.410  Farmington                                                                       444.55      Bristol

145.450 South Bristol                                                                      444.700    Northern Rochester

146.610  Rochester                                                                          444.800   Fairport

146.640 Weathersfield                                                                   444.850    Rochester

146.655 South Bristol                                                                     

146.685 Sodus

146.715 Perinton

146.745 Newark

146.760 Rochester

146.79   Rochester

146.82   Canandaigua

146.88   Rochester

146.925 Rochester

147.075 Henrietta

147.180 Macedon

147.225 Brockport (just barely)

147.375 Rochester

Conclusion and Summary:

I learned, honestly, quite a lot from this little project and have a light, solidly constructed, weather sealed,  capable, dual band 2m/440 antenna for long term use that hits all local repeaters.   When Chris again wants to keep ups moving forward as a club using the WB2KAO 70cm repeater, I will be there.

Next little project:  Integrate the model for this little antenna into the overall model for my entire antenna system and see if interaction with the steel mast and the rest of the wire in the air results in the SWR falloff noted in the lower part of the 70cm band. 

 

Mike

KM2B2021-10-23
Re: Covid-19 Dual Band 2m/70cm Antenna Build
ok, the wire diameter is 3/32". Hence, I divided by 2.

Sorry for not being clear in former post.
Reply to a comment by : KM2B on 2021-10-23

KD9PCM: Yes, I used the radius of the wire which is 3/32" = 0.09375/2. In the model I have a GS card converting inches to meters. Also, in NEC2 you specify wire conductivity with the LD card. In this case I looked up brass wire conductivity and entered in in mhos. Thanks.
Reply to a comment by : KD9PCN on 2021-10-23

Great article! You didn't specify how you modeled the brass conductors in 4NEC2. I presume you specified brass (is that available in 4NEC2?) of radius 1/2 of 0.09375", is that correct?
KM2B2021-10-23
Re: Covid-19 Dual Band 2m/70cm Antenna Build
KD9PCM:

Yes, I used the radius of the wire which is 3/32" = 0.09375/2. In the model I have a GS card converting inches to meters.

Also, in NEC2 you specify wire conductivity with the LD card. In this case I looked up brass wire conductivity and entered in in mhos.

Thanks.
Reply to a comment by : KD9PCN on 2021-10-23

Great article! You didn't specify how you modeled the brass conductors in 4NEC2. I presume you specified brass (is that available in 4NEC2?) of radius 1/2 of 0.09375", is that correct?
KD9PCN2021-10-23
Covid-19 Dual Band 2m/70cm Antenna Build
Great article! You didn't specify how you modeled the brass conductors in 4NEC2. I presume you specified brass (is that available in 4NEC2?) of radius 1/2 of 0.09375", is that correct?
KM2B2020-11-10
Re: Covid-19 Dual Band 2m/70cm Antenna Build
W5TAO:

I just saw this comment. Question: With the ferrite in place did what frequencies did you test? The antenna works on both 2m and 70cm band but I would be interested in the specific frequencies you tested.

Thank you.

KM2B
Reply to a comment by : W5TAO on 2020-07-02

I made one of these antennas. I tried a ferrite choke on the vertical and moved it up and down the vertical while watching the SWR. I got a 1.12 SWR with the ferrite about 1" from the top.
KM2B2020-07-14
Re: Covid-19 Dual Band 2m/70cm Antenna Build
KE8BFN:

Yes, my source was onlinemetals.com. Shipping was $18.

However, I ordered quite a few rods.
Reply to a comment by : K1KIM on 2020-07-14

Is your source for the rods Onlinemetals.com? I found their shipping to be extremely high.
Reply to a comment by : KM2B on 2020-07-01

KJ4DJE: Thanks for reading. It was, indeed, great fun to do this little project. In fact, up here in Rochester, it was a very cold spring, so, I was still indoors a bit more than my liking. This project took a few days all things considered and kept me active, interested and engaged, and, worked better than I thought it might. I learned a bit too, about antenna's simulation, which I spent a lot of time on first, and simple builds and found a great source for brass rod. '73
Reply to a comment by : KJ4DGE on 2020-06-24

Excellent article and great pictures. I made similar type antenna and they work well. The fun is in the exploring, the building and satisfaction when it works well. Keep the hobby alive and build something from scratch. This is what HAMs did before HRO :)
K1KIM2020-07-14
Re: Covid-19 Dual Band 2m/70cm Antenna Build
Is your source for the rods Onlinemetals.com?
I found their shipping to be extremely high.
Reply to a comment by : KM2B on 2020-07-01

KJ4DJE: Thanks for reading. It was, indeed, great fun to do this little project. In fact, up here in Rochester, it was a very cold spring, so, I was still indoors a bit more than my liking. This project took a few days all things considered and kept me active, interested and engaged, and, worked better than I thought it might. I learned a bit too, about antenna's simulation, which I spent a lot of time on first, and simple builds and found a great source for brass rod. '73
Reply to a comment by : KJ4DGE on 2020-06-24

Excellent article and great pictures. I made similar type antenna and they work well. The fun is in the exploring, the building and satisfaction when it works well. Keep the hobby alive and build something from scratch. This is what HAMs did before HRO :)
KO4DNI2020-07-05
Re: Covid-19 Dual Band 2m/70cm Antenna Build
Great job. And an interesting read. Thank you for sharing this. KO4DNI
Reply to a comment by : KJ4DGE on 2020-06-24

Excellent article and great pictures. I made similar type antenna and they work well. The fun is in the exploring, the building and satisfaction when it works well. Keep the hobby alive and build something from scratch. This is what HAMs did before HRO :)
KM2B2020-07-03
Re: Covid-19 Dual Band 2m/70cm Antenna Build
W5TAO:
Thank you for the effort to build the antenna. Your time is appreciated.

What frequency were at when your SWR measurement was made with the ferrite near the top?

I had not thought about using a ferrite to locate the optimal length relative to the actual length. I will try that as well.

Thank you.
Reply to a comment by : W5TAO on 2020-07-02

I made one of these antennas. I tried a ferrite choke on the vertical and moved it up and down the vertical while watching the SWR. I got a 1.12 SWR with the ferrite about 1" from the top.
W5TAO2020-07-02
Covid-19 Dual Band 2m/70cm Antenna Build
I made one of these antennas. I tried a ferrite choke on the vertical and moved it up and down the vertical while watching the SWR. I got a 1.12 SWR with the ferrite about 1" from the top.
KM2B2020-07-01
Re: Covid-19 Dual Band 2m/70cm Antenna Build
KJ4DJE:

Thanks for reading. It was, indeed, great fun to do this little project.

In fact, up here in Rochester, it was a very cold spring, so, I was still indoors a bit more than my liking. This project took a few days all things considered and kept me active, interested and engaged, and, worked better than I thought it might.

I learned a bit too, about antenna's simulation, which I spent a lot of time on first, and simple builds and found a great source for brass rod.

'73
Reply to a comment by : KJ4DGE on 2020-06-24

Excellent article and great pictures. I made similar type antenna and they work well. The fun is in the exploring, the building and satisfaction when it works well. Keep the hobby alive and build something from scratch. This is what HAMs did before HRO :)
KM2B2020-07-01
Re: Covid-19 Dual Band 2m/70cm Antenna Build
K5TEE:

Thank you for reading. I just noticed it was posted here.

The 2m repeaters in my area are in the region of the band where the SWR is increasing by decreasing the length of each element.

As an overall compromise, this was OK so I left it and wrote this up.
Reply to a comment by : K5TEE on 2020-06-26

I wonder what your results would look like if you shorten the antenna by another 1/4 to 1/2 inch. Look at your graphs, it shows the antenna is still too long. Tom
K5TEE2020-06-26
Covid-19 Dual Band 2m/70cm Antenna Build
I wonder what your results would look like if you shorten the antenna by another 1/4 to 1/2 inch.
Look at your graphs, it shows the antenna is still too long.

Tom
KJ4DGE2020-06-24
Covid-19 Dual Band 2m/70cm Antenna Build
Excellent article and great pictures. I made similar type antenna and they work well. The fun is in the exploring, the building and satisfaction when it works well. Keep the hobby alive and build something from scratch. This is what HAMs did before HRO :)