I thought I would share a few of my notes on tuning up a 6BTV antenna, and constructing a radial field. I have been using a GAP Challenger DX, (review here), for the past fifteen years, unbeknownst to me, and over that time, water had been slowly seeping into the tuning harness of the antenna.
The harness finally gave up the ghost one day, presenting a dead short to my Solid State ALS-1306 amp, (review here), as I increased power. At about 800 watts CW, the harness shorted out, killing the ALS-1306, and necessitating the amp make a trip back to ALS-1306 heaven, (Ameritron), to have a replacement RF output device installed.
Replacing the Challenger was one of the options on the table, but I did not want to do a new install of a Challenger– I remember the last install. The entire phasing harness installation process is not fun, nor is it easy, so– given there were a few things about the Challenger I was not happy with anyway, I began looking for a different antenna.
A used 6BTV came up on the local market. I knew the history of the antenna, and I had known the owner for decades, so I knew the antenna had not been abused. I also knew that the antenna was less than a year old. The price was right, so I purchased it. It arrived at my QTH with a cracked trap cover for 10 Meters, (I assume this happened in transport), and with lot of water dripping out of the 20 meter trap. I ordered replacement trap covers from New-Tronics, and proceeded to replace all the trap covers once the new ones arrived. Thankfully the previous owner had used Pentrox-A on all sliding parts, so the antenna came apart like butter. I also noticed that one of the hose clamps used to tighten the 10 meter element was stripped, so I got a new one of those as well. This pretty well took care of getting all the loose parts in one place in preparation for tuneup of the 6BTV. You can click any image on this page and get the original in large format.
I am one of those people that likes to read all about a process before I start, so in my wanderings around the net I found out a few things about the 6BTV:
- The factory configuration and spacing diagrams for tuning are for no radials.
- DX Engineering has a wonderful install guide for the 6BTV which puts the manufacturer’s instruction guide to shame.
- There are a lot of additional add ons DX Engineering sells for the antenna, all good.
- I would need a radial field to realize better performance out of the antenna.
- I was not happy with the manufacturer’s instructions to tune the antenna for the center of the band. I do mostly CW, or data, so that would not do.
The first thing I discovered was that I wanted to do this by the book, and do it right, so I constructed a step by step process guide to do the install and tuneup.
One of the first items was to decide where exactly to install the multiband antenna, and have it not affect other antenna, or be affected by other antenna. I have plans for a four square on 30 Meters one day, so I decided to draw out the final configuration for a two element 40 Meter phased array, and a four square on 30 Meters. I obtained a plat map from the county, and added the antenna and radial fields for all future antenna. This helped me select the right location for the 6BTV. It is the antenna just to the East of the tree in the backyard. I also installed underground cable feeds, and a remote coax switch on the rear of the work shed, and then ran a single feedline to the house and the shack.
Next I constructed a step by step guide with all the steps I needed to take to get the 6BTV up and operational.
- Install support for antenna
- Install antenna
- Construct Radial Field
- Perform very rough tuneup on the antenna.
- Measure R value at base of the antenna.
- Add radials, and remeasure the R value.
- If R value makes no major change from the previous reading exit to step 8, else goto 6.
- Define the operating frequency for each band.
- Measure actual two to one SWR bandwidth for each band.
- Select the center of the two to one spread, and decide where to put it, so that I could operate in the lower parts of the band, and cover the data parts as well.
- Tune the antenna to put the center point for a two to one SWR at the correct location on the band to give me no less than one point five to one SWR at the bottom of all bands but 80 Meters.
- Put 80 Meters resonance point at around 3.800 MHz, so I can use the antenna on the phone portion of the 80 Meter band.
Install Antenna Support Structure:
The very first thing I did was to call in a locate for all pipes, and cables that might be under my yard. This step absolves you of all liability if you hit something that you should not hit while digging. I NEVER dig without a locate.
I next located where each hole needed to be, and then I marked each location with a red circle. Also given that I had a large number of post holes to dig, I elected to use a powered post hole digger, which I rented from one of the local rental shops. The thought of digging each each post hole with a hand held post hole digger was not appealing to me at all, and I am not sorry for the money it cost to rent the digger.
I made each hole eight inches in diameter, by two feed deep. After taking maybe two hours to dig all holes, I returned the post hole digger to the rental shop. After policing up the dirt, and moving it to low spots in the field, I next dropped about an inch of gravel into each hole. The gravel keeps water off of the concrete, or at least it is supposed too…
I next purchased a set of six pole levelers. They are cheap, only a few bucks, and once you have the posts in the concrete, you want to quickly be able to see if the posts are vertical. If not, you can tap them into place, prior to the concrete setting. I also wanted a ground rod at each antenna site, so I again headed to the local rental store. I rented a ground rod driver. This is an impact driver and it takes about 10 minutes to drive in an eight foot ground rod into gravel and clay. I have driven in ground rods before here, and that takes about an hour, six of these would take far longer as I would tire out. I just rented the driver, and spent about an hour and drove them all in.
After allowing the concrete to cure for about a week, I installed the DX Engineering Radial Plate, and the DX Engineering Tilt plate for the antenna. Next I bolted the antenna to the tilt plate, and connected the existing five radials from the old Challenger install to the radial plate. I used eye rings, to do this, crimping each ring carefully as they would be out in the weather for years to come.
The Challenger had no need of a radial field, save the three radials used as a counterpoise for 40 Meters. So I repurposed the three radials, and added 5 more. That gave me 8 radials, as 40 Meters was the lowest frequency band I had any real interest in, I made them all 1/4 wave long on 40. The goal is to add as many radials as needed to see a very small change in R while I double the radial count. When I see little change, or if I approach 36 ohms, I will stop, as the next doubling will do even less than the previous doubling did to reduce R.
Next I did a very rough check of the tuning of the antenna, making sure it was in the ham bands, I walked in each band using the AA-54 to check SWR, and as I got to each band, I just got the dip into the band, not at a particular location within the band. That would come later.
As the radial field changes, the antenna tuning changes, so there is an interaction between the two. I did not want to spend a few hours getting the antenna tuned to the exact spot in a band, only to have that tuning change as I added radials, or chokes.
Measure R at base of antenna:
Next I dropped into loop mode, measuring the R value then I adding radials, and measuring R again. I ended up with 13 radials, and will probably make it 16 radials just because 16 is a power of 2, and numbers that are not bother me… I started with 4 radials, measured R, then added 4 more, that dropped R, I added 8, and R dropped again, although not very much. I added another 5 radials, (ran out of material), and measured R again, and saw almost no change… So I concluded doubling again would not win me much lower R values, and called it done! I am a bit surprised at how fw radials it took to get to this point, and I will be adding more as time goes on anyway, because I believe I will get a bit more performance, but not much beyond 32.
Define the operating frequency for each band:
Now that the radial field was mostly complete, I could actually begin the carefully tuning the antenna. I first decided in each band where I wanted the low point to be for SWR. I operate CW more than any other mode, so I wanted to skew all SWR readings to the low end of each band, save 80 Meters– I only do SSB on 80, so I wanted that to be on the high side of 80. To do this I needed to decide what the bandwidth of the antenna was across my arbitrarily selected worst case SWR conditions for each band.
I have an ALS-1306 solid state amplifier, and solid state devices do not like high SWR, so I decided that 1.7 to 1 was the worst case I wanted. I then took whatever the bandwidth was between the low 1.7 to 1, and the high 1.7 to 1 SWR points on each band, 80 through 10, and subtracted the high frequency from the low frequency, that gave me my available bandwidth for every band. After dividing the bandwidth in half, and adding it to the low end of each band, I had the location I needed to place the low point for SWR for each band. I ended up with the numbers below:
- 80 Meters: 3.910 MHz.
- 40 Meters: 7.133 MHz.
- 30 Meters: 10.122 MHz
- 20 Meters: 14.000 MHz.
- 15 Meters: 21.085 MHz.
- 10 Meters: 28.400 MHz.
This would give me between 2.00, to 1.7 to 1 SWR at the bottom of each band. The upper end was left to fall wherever it ended up, in most cases the top end fell past the high end of each ham band. 80 Meters is the only exception, and is treated as a special case. You will find below the final values for all ham bands, and in most cases the starting point. Once we hit 15 Meters, the bandwidth is so wide as to cover the entire band, plus a wide margin, so that data is not included. If you are interested in that data, please feel free to email me, I will send you the raw data files.
Below you will see six images, one for each band, 80, 40, 30, 20, 15, and 10 Meters. Each image consists of four graphs, from top to bottom the graphs represent the following, SWR as viewed at the feedpoint, R, X, and Z, as viewed from the feedpoint, SWR as viewed from the shack, and R, X, and Z as viewed at the shack. You can click a graph to get an expanded view, click the expanded view and you will get an even larger view.
Information on additional radials:
I took a set of scans of the SWR of the antenna using 8 radials, and then again using 20 radials. The result was that for the most part as radials are added the resonant freqiency shifts upwards slightly. The recieved signal levels however increased dramaticly… On average most signals were around S4 to S5 in the FT8 sub-band. As redials were added, I noted an increase in overall levels of around 3 to 3 S units on the K3. The S readings were taken over a few days time, then averaged out to get to that number. Here are the SWR grapsh of the same antenna, in the same place, with the same ground conditions, (no rain), as the radial count went from 8 to 20 radials.
Click a graph for an expanded view
One must remember in this scenario, that a 1:1 SWR on a vertical guarantees it is not set up correctly. A perfect vertical, in a perfect environment will have around 36 ohms, and show a 1.4:1 SWR. Rather than try and explain why, see this very extensive article on the subject.
Other graphic information on the 6BTV:
Click a graph for an expanded view
Several people have asked what happens as radials are added… I dug out my old antenna files from the Rig Expert Analyser. It turns out I captured the antenna data as I added radials. So, that said, here are the graphs for SWR going from 8 radials, to 24 radials. All measurements were taken using a RigExpert, AA-54 analyzer. I am finishing up a review of the AA-54 and expect to have it up on site within a few weeks. I am most impressed with this device, it is a real instrument, and makes antenna work fun! Please feel free to comment on the installation here, as I am seeking all input, positive or negative…
Click on any image above to expand.