I am building the 3-element 80 meter vertical yagi in stages. The first stage is the driven element, the greatest part of which is a ground isolated tower. This is the very same DMX-52 tower and floating base that I used at my previous QTH to support a tri-band yagi, wires and was fed as an 80 meter top loaded vertical (yagi as top hat capacitor).Since the tower is ~14 meters a stinger is needed to take the antenna to a full λ/4 on 80 meters (~19.7 meters in my case). The stinger is made of aluminum pipe and tubing with the structural strength to support the wire parasitic elements. To maximize the vertical height of the wire parasites (for best performance) I added one meter of PVC pipe on top. Anything longer would become unwieldy and less robust. I want this antenna to last.
Construction, tuning and testing of the array is a large enough project that one article would be impractical so it will spread over several. Even if this array is of no interest there should be aspects of interest to any ham with an interest in building antennas and towers. If nothing else this first article may be of interest to those putting up small towers and low band ground-mounted verticals.
Tower base
My first task was to choose a site. After various considerations it ended up very close to where it was in my original site plan. The location has these attributes:
- Well spaced from power lines (50 m), Beverage antenna field (30 m) and existing towers (60m and 70 m), while not being too far from the shack (60 m). My major concern is interactions in a couple of directions. From what I have read and from other hams with similar issues I expect my choice to work pretty well, with no enhanced minor lobes (F/B, F/R) and gain to the southeast reduced well below 1 db.
- Minimized impact on haying. Radials and support ropes preclude farm equipment and could take ~1 acres out of production. By moving closer to the tree line the radial system overlaps the perimeter bush, thus reducing tillable land impact by ~15%.
- Ease of access for mowing and other regular maintenance.
When the snow melted I resumed work on the base. I used a similar method as before for sitting the tower legs on the wood base but took additional step to ensure good RF isolation from ground. A thick plastic block is placed under each leg and bolted to the base. This first requires accurate siting to the guy anchors. Not visible is a ½" length of rubber tube that pierces the plastic block and L-bracket. A rubber grommet sits atop that and a screw lightly holds it all down.
The base does not prevent the tower from overturning; that's the job of the guys. The rubber allows a small amount of rocking in high winds, prevents lateral motion and electrically isolates the tower (driven element) from ground. The driven element will be directly fed between radials and tower.
Raising and guying the tower
As I've mentioned a couple of times I ran into an apparent problem with the ground anchors late last fall and decided to be prudent and stop construction until I could address the problem in the spring. That time has come, adjustments were made and construction resumed. I'll review the problem and how I decided to proceed.
Since the load on the tower and antenna array is modest I decided to use ordinary augur-style anchors. These have a bevelled blade at the bottom that acts as both a bit and as a load bearing surface. It is best to screw them in with a augur attachment on a tractor but it is possible to do it by hand with some effort. I don't have a tractor and it seemed excessive to rent one or cajole a neighbour to help so I did it by hand.
I don't include a picture of the anchors since I neglected to take one before burying them. For a ham relevant discussion of screw anchors, and pictures, I'll refer you to W8JI. The only difference is that mine are much shorter at 3' (~90 cm) long, common in farm country to anchor fence lines. That page has related information I'll come to shortly.
After carefully siting the anchors so that they are precisely 120° apart and 12 meters from the base of the 14 meter tower I broke the surface sod with a shovel. Other than hitting rock the surface is the most difficult to penetrate since vegetation roots in soil form a surprisingly solid mass. If done carefully the sod can be put back once the anchor is in place.
The tools required are quite simple: shovel, steel bar and a sledgehammer. The steel bar should be long enough for turning torque and to push down against but not so long that it hits the ground every half turn. I used my old trusty 1" cold chisel.
Quite a lot of force may be required depending on the soil type. Even if the soil is not hard you must still press down hard as the screw is turned or the soil will be ground up and weakened until time eventually heals the wound. Minimizing soil disturbance is perhaps the most important reason to use a power augur to drive in screw anchors. When a stone interfered with progress a judicious tap of the sledgehammer on the anchor pushed it aside just enough to screw past it.
Overly disturbed soil is what stopped me in the fall. Two of the anchors had 1" to 2" freedom of axial movement in the waterlogged soil after being screwed in. Since I couldn't tell whether this was temporary or my 3' anchors aren't long enough for the soil type I elected to wait until spring to decide whether to fit the anchors in concrete to form a larger soil bearing surface.Once the ground frost was sufficiently thawed (tested with a soil probe) I tested the anchors and none moved under load. However I partially unscrewed and redid one that I had put in at too shallow an angle. I was aiming for ~45° since the guy station is up the same distance the anchor is from the base: ~12 meters.
Newly confident that the anchors would hold I manually stood the two pre-assembled bottom sections, completed the base (see above) and temporarily guyed the tower with ropes and turnbuckles. When I reached 4 sections (~31') I attached temporary steel guys and one-by-one shifted the load from the ropes to the steel guys. I did this by loosening the turnbuckle, slipping on the steel guy over the open hook termination of the turnbuckle screw, tightened the turnbuckle and finally removed the rope.
Do this methodically or you risk the tower toppling. It only takes a few minutes so don't become careless from impatience. The picture shows the final rope guy about to be replaced. For additional safety I placed a ~100 lb stone on a lumber cradle sitting over the bottom X-braces of the tower.
Sections went up very quickly using the same gin pole used before for this tower. Since the original aluminum angle was claimed by another project I replaced it with steel angle stock from my junk pile. The gin pole worked well despite its limitations.
There was a two week delay topping the tower due to a series of late spring snow and ice storms, the need to keep the top section at hand for constructing the stinger, and to recover from a wisdom tooth extraction. It was very frustrating. When work could resume I raised the top section with stinger attached, retracted into the section so that it was not too top heavy for lifting and splicing. Lifting and inserting the stringer separately would have been awkward and potentially dangerous due to it length.
 |
| Topped; still nested & temp guys |
About 1 meter of PVC pipe at the top supports a guy ring and rope catenaries for the wire parasitic elements. The ropes must be attached at this time since the top of the stinger is out of reach once it and the top tower section are raised. The ropes are lightly tied to the top section until the wire elements are installed later.
Raising the top section complete with nested stinger was more of a problem than expected. It's slightly top heavy and the improvised gin pole couldn't grab it any higher. Tag lines were used to direct it around the temporary guys and then to pull it roughly vertical so that it could be spliced. Despite all the problems the entire operation took only 2 hours.
With everything up the permanent steel guys are attached and the temporary guys removed. The guys are a combination of ⅛" and 3/16" aircraft cable salvaged from their first use on this tower. The top segment is kept very short to minimize capacitive loading. The other segments are non-resonant on 80 meters and have negligible loaded per my EZNEC model.
I originally intended to guy with the black dacron rope I bought for this purpose. Instead I went with steel to reduce deflection of the structure in high winds which could stress the base and parasitic element wires. The rope will go to one of a couple of projects tentatively planned for the future.
The tower feels very solid and survived 90 kph wind when held with the second set of temporary guys at 40'. Even the unrestrained stinger did fine. I don't anticipate a problem when complete. The anchors and guys will be regularly checked for the next few months to ensure that the anchors are not shifting, especially after heavy rainfalls which can partially liquefy the topsoil.Radials
The radials are attached using a similar arrangement to the one I improvised for the 160 meter t-top vertical. It worked so well and is inexpensive and easy to use I couldn't resist. We'll have to see how it survives in practice.
The attachment pillar is a 3-½" plastic coupler friction fit over several screws driven into the floating base. An all stainless steel (the band and the screw) hose clamp secures the radial wires and the wire to the feed point. Gripping the copper conductors between an insulator and stainless steel greatly reduces the risk of galvanic corrosion. The large diameter pillar is helpful when the radial count is high and it minimizes the deflection when a radial has to be routed around a tower leg.
To attach a radial you simply strip 1" of wire, slightly loosen the hose clamp, slip in the wire and fold it over the band. Tighten the hose clamp and it's done! For the initial test (first light) there are 4 x 20 meter long radials. All radials are AWG 18 solid insulated wire. The price is reasonable and is more than adequate for QRO when many radials split the antenna current.
First light
As first tested with my analyzer the resonance was not as expected. Resonance is almost exactly 3.4 MHz with an impedance of 50 + j0 Ω, well below the expected resonant frequency of 3.9 to 4.0 MHz with the stinger partly nested inside the top section. (If you expand the photo above you should be able to see the SWR curve on the analyzer screen, which is centred at 3.4 MHz.)
This appears to be due to the small number of long radials currently installed and perhaps I did not properly account for the tower diameter in the model. Once the situation has been fully investigated remedial measures will be taken. The impact of the former item per EZNEC is to lower the resonant frequency by 100 to 125 kHz since the on-ground radials are longer than an electrical ¼λ and, due to the low count, greatly affect resonance
Although the SWR of 1.0 looks very nice it is not. Recall that the radiation resistance of a ground mounted ¼λ vertical is typically 35 to 37 Ω, and can be lower for a "fat" monopole such as mine. The ground loss, which is in series with the radiation resistance, is therefore approximately 15 Ω. That's quite high although entirely typical for the small number of radials currently installed. As radials are added the feed point resistance will fall.
My objective is no more than 5 Ω so that ground loss is a minor factor when the array is in full operation as a 3-element yagi, whose radiation resistance is much lower than a vertical alone. This is modelled and explained in more detail in the antenna design article.
About that stinger
The final stinger height must be firmly determined before the parasitic wire elements are designed and installed since the resulting geometry determines the structure of the t-top parasitic wire elements: lengths of the vertical leg and t-top. There are also mechanical considerations I must deal with.
The driven element does not absolutely need to be resonant. The feed point resistance will be low enough when all the radials are in place that an L-network may be desirable to lower the SWR when used in the array's omni-directional mode. Provided that the resonant frequency is roughly in band that will ensure sufficient mutual coupling with the parasitic elements for the array to perform as intended.
The final call on stinger height will come after initial testing and a little bit of modelling to verify adjustments to the design.
Getting from here to there
With the basics done I have run coax to the the antenna so to compare the vertical to the temporary inverted vee up at 32 meters. I will write up the comparison for the blog. The comparison will help to establish baseline performance to a known antenna. Once that's done the inverted vee comes down to get it out of the field for haying season. I will almost certainly put it up again to work nearby US stations in contests, with height and location to be determined.
The permanent feed point will be constructed, the stinger redone and L-network designed. Tuning requires completion of the feed point since the temporary setup will certainly have different wire lengths from the coax termination to the radial hub and to the tower (monopole). At least 16 radials will be required to erase most of the resonance-influencing effects of the radials. Again, I'll delve into this in a coming article.
With all that out of the way the parasitic elements will be hung off the driven element stinger, tuned and radials laid. As you can see there is a logical sequence of steps to go through when building an array of this nature.
Before the switching system is fully deployed I may temporarily wire it as a fixed yagi to assess performance. I will then need to complete and deploy the direction switching system, switchable L-network and switching units for the parasitic elements. All of this is straightforward but time consuming.
Unless other projects deflect my attention over the next few months the array could be substantially complete this spring. The final objective is that it be complete in time for the fall contest and DX season. That's when I find out for sure how well it performs. Either way it is going to be interesting!
No comments:
Post a Comment
All comments are moderated, and should appear within one day of submission.