Multiband Vertical Antenna - Part Deux......

[marty_wittrock]

It's been a busy summer at the QTH in central Iowa but with thoughts of a upcoming (if it will ever get here) sunspot cycle, fall camping, and the prospect of a new mobile HF rig, I've been thinking of applying the design of the original Multiband Vertical Antenna http://chat.qth.com/viewtopic.php?t=4455 to a mobile application, using the matching transformer and a long (possibly 54") slim vertical whip and mounting that to either to a magnetic mount or frame (near the bumper) mount on my Honda 2-seater. The idea struck me while I was driving my 45 minute commute out to work and it seemed like a feasible idea. The original design required no radials and was completely autonomous - - so why not apply the same idea to a mobile antenna? The idea of a Multiband Vertical Mobile Antenna was born.

Be looking for the updates over the next 4 weeks - the original antenna worked so well that I think applying it to a mobile application should be no different and work just as well for the 100W-class rigs. I will also be posting pics and the same text on my website http://www.kn0ck.net too......more to follow on that.

Stay tuned.......73's de Marty, KN0CK

[marty_wittrock]

Since I last wrote I commuted to work each day thinking of what the antenna design should be comprised of to keep it within the same rules as the Multi-Band Vertical I use on my home Ham station. My homebrew wideband antenna at my home fixed-station was entirely comprised of common home-builder store components: aluminum pipe used for decoration or utilitly, some minor copper fittings to 'plumb' the antenna together, house wire (AWG 14), and common PVC plumbing fittings found on the shelf. Everything except the ferrites (purchased from Newark and shipped from the U.K.) came from our local Menards home builder store - - and the mobile antenna would turn out to be no different......keep reading......

I used the exact same transformer design as I used before on my fixed-station antenna. So there was no difference here. Refer to http://g8jnj.webs.com/broadbandhfvertical.htm for more info on the transformer assembly and http://chat.qth.com/viewtopic.php?t=4455 for the trials of the fixed-station antenna blog that I wrote back in March of this year (2008). Order info for the ferrites I got from Newark is in that blog, too.

About a week ago I got on R&L's website and decided that the base for the mobile antenna needed to be a magnetic mount such that it wouldn't ruin the finish (or the looks) of my red Honda 2-seater. I ended up purchasing a mag mount that is 6" in diameter (beefy!) and has a SO239 connector in the middle to allow the transformer assembly to reside (once again) within a 2" PVC pipe housing. I was at Lowes (since I was in the neighborhood) and found a 2" to 1/2" adapter that ended up being a perfect fit to the PL259 connector that would be JB-Weld-ed to the 1/2" threaded hole on this PVC adapter. Before I glued it down, I used a propane torch (at a low setting) to solder the outer shell of the connector (threaded part of the PL259) to the body of the connector to keep the PL259 rigid. Once that was done I added a 2" length of 14AWG wire from the back shell (ground) of the connector to allow connection to the ground lead of the transformer assembly. I then took a 3" piece of 14 AWG wire and soldered it to the center conductor of the PL259 to allow connection to the driven part of the transformer. Taking the completed PL259 in hand, I mixed some JB-Weld and applied it to the outer shell (knurled) part of the PL259 and inserted it into the 1/2" hole of the 2" to 1/2" PVC adapter. As well, I applied a pretty thick coat of JB-Weld to the other side of the PVC adapter to really lock the PL259 in place. That made for a REALLY nice looking connection that will thread FB to the SO239 on the magnetic base.

I then obtained a slightly rounded 2" PVC cap and drilled a 5/16" hole to allow a 3/8" x 24 thread coupling to be threaded onto the top of this cap. The idea was to use a 3/8" x 24 thread antenna on the top of the transformer assembly to provide the necessary electrical length needed to radiate and provide the load to the other side of the transformer. Underneath the 2" cap, I used a 5/16" crimp ring connector and crimped/soldered the antenna part of the transformer to interconnect to the 3/8" x 24 stud. The cap will eventually be connected to a small (about 3" long) 2" PVC pipe and glued. The other end of this pipe will contain the transformer assembly and that will be connected to the PL259 to finish the electrical connections. A 2" PVC coupling will connect to the base assembly and also connect to the cap section to fully enclose and hold the transformer assembly. It'll also be completely waterproof.

It's my intention to use a standard off the shelf HF vertical 3/8" x 24 thread whip antenna for the main radiator of this mobile antenna. It would be ideal if it were a 15m or 10m electrical length to match what I have in electrical length of the fixed-station antenna, but as it is all I have is a 80m wound whip - - so I'm going to start with that. If my VSWR is crummy with that antenna for some reason, I'll end up making a 16' electrical length whip with a wire-wound base and a remnant spring steel whip off of some old HF antenna laying in the garage. I'll keep you posted with how that fares. With 16' of electrical length, the fixed-base antenna had excellent VSWR performance from 80 - 10m, so I don't expect that the mobile will be much different. As I mentioned at the outset of this series, the transformer-coupled design lends itself well to a completely autonomous antenna design - very similar to how it's used here at the fixed-station antenna and works fine for a non-radial antenna. So the mobile design will be very similar in practice, and be wideband, too.

Pictures of the separate assemblies and the final construction will be posted to my website within the next 36 to 48 hours, along with the test data in the form of a VSWR plot from 80 - 10m. As I write this, the antenna components have been glued and are curing, all the parts have been fabricated, and final assembly will commence tomorrow morning - - tests to follow shortly after that.

Stay tuned.......73's de Marty, KN0CK

[marty_wittrock]

I got up this morning and immediately went to work getting the Wideband Vertical HF Mobile Antenna assembled and first VSWR tests completed with the 75m vertical antenna threaded to the top of the antenna's base loading transformer. I found during assembly that I should have recessed the PL259 deeper into it's PVC mounting base such that the base would sit flush to the magnetic mount. As it is now, there's about a 1/4" gap between the bottom of the transformer base and the magnetic mount when the PL259 is completely threaded to the mag mount. Oh well, looks like that's another trip to Menards to look for something to seal the gap (a thin rubber 'puck' or something like that). Lesson learned: make sure you gauge the depth of the PL259 before gluing it in if you want the base to look right. Also, most PL259 connectors have a couple of small 'prongs' at the barrel edges of the connector body to mate with indentations of the SO239 to prevent the PL259 from moving. You'd be well advised to remove those 'prongs' from the PL259 (file them off prior to gluing it to the PVC base) or else you'll be challenged to make the connector completely seat on the SO239 - - in my case I ended up using a Dremel tool (post-mortum) to remove those 'prongs'. Another lesson learned.

Assembling the antenna, I soldered the two connections remaining between the transformer and the PL259 (making sure that I had all the PVC lined up to assemble since this was the final time those parts would be separated). Soldered the connections and taped them for coverage, began assembling the PVC pieces together and glued them all with PVC cement as I went along. The final thing to be assembled was the cap to the rest of the assembly - I wound the remaining lead-length from the antenna connection within the cap and then applied the PVC cement. No turning back now - this would be the last time the transformer would see the light of day. Final assembly looked SHARP (albeit with the PL259 not being gauged right). I liked the style of it even if it was comprised entirely of PVC plumbing parts. It looks REALLY COOL (for an antenna geek like me)! I may even paint the body of the transformer case to match my car...!

I took the completed base and the 75m whip to my Honda 2-seater. Put the beefy mag mount in the middle of the truck lid and then rolled down the rear window of my car to allow the cable to pass through it temporarily for this test. I then connected up my Daiwa VSWR meter to my FT-817 and set the level to 0 -10W range for VSWR measurements (recall, the FT-817 is a 'pip-squeak' with power at about 10W PEP SSB). Threaded the 75m whip on top (now it even looked more impressive!). Powered up the rig, dialed-up 1.8MHz and began the series of VSWR tests from 160m - 10m as follows:

1.8 MHz = 4.0:1
1.9 MHz = 4.0:1
1.99 MHz = 3.5:1
3.5 MHz = 2.1:1
3.6 MHz = 2.1:1
3.7 MHz = 2.0:1
3.75 MHz = 1.8:1
3.8 MHz = 1.6:1
3.9 MHz = 2.2:1
3.970 MHz = 3.0:1
7.0 MHz = 1.8:1
7.1 MHz = 1.8:1
7.2 MHz = 1.8:1
7.249 MHz = 1.8:1
10.1 MHz = 1.8:1
10.15 MHz = 1.8:1
14.0 MHz = 2.0:1
14.1 MHz = 2.0:1
14.2 MHz = 1.9:1
14.25 MHz = 1.9:1
14.3 MHz = 1.9:1
14.35 MHz = 1.9:1
18.075 MHz = 2.0:1
18.1 MHz = 2.0:1
18.15 MHz = 2.0:1
18.165 MHz = 1.9:1
21.0 MHz = 1.5:1
21.1 MHz = 1.5:1
21.2 MHz = 1.4:1
21.4 MHz = 1.5:1
21.3 MHz = 1.3:1
21.4 MHz = 1.3:1
21.45 MHz = 1.3:1
24.890 MHz = 1.1:1
24.950 MHz = 1.1:1
24.990 MHz = 1.1:1
28.0 MHz = 1.3:1
28.1 MHz = 1.3:1
28.2 MHz = 1.3:1
28.3 MHz = 1.3:1
28.4 MHz = 1.3:1
28.5 MHz = 1.4:1
28.6 MHz = 1.4:1
28.4 MHz = 1.4:1
28.5 MHz = 1.4:1
28.6 MHz = 1.4:1
28.7 MHz = 1.4:1
28.8 MHz = 1.4:1
28.9 MHz = 1.4:1
29.0 MHz = 1.5:1
29.1 MHz = 1.5:1
29.2 MHz = 1.5:1
29.3 MHz = 1.5:1
29.4 MHz = 1.5:1
29.490 MHz = 1.6:1

WOW!! I was amazed at the performance of this antenna!! I receives just like it does in the house and all bands srang to life with signals when I was running through testing the VSWR tests. I wasn't too surprised with the fact that the 160m band was REAL high VSWR - that was consistent with the fixed-station antenna. But when I was measuring 80m - 10m, the VSWR reminded me A LOT of the performance of the fixed-station antenna. At 75m (where the whip is resonant) I noticed its VSWR curve, but was happy to see where it wasn't resonant the VSWR was kinda high, letting me know that the transformer was trying to help out (the VSWR was 3.0:1). So I may try to set the whip to the spot on the 75m band that I work the most to optimize that, but it was refreshing to see that the whip was really getting used and the matching transformer wasn't absorbing the power that much (if at all) in that part of the band. One other issue that I didn't see when working this antenna that I have seen in the 'multiband' HF mobile antenna I also use (Perth Plus) is that the FT-817 didn't indicate 'HIGH VSWR' anywhere where I tuned up to test the VSWR. So the matching transformer must be providing a pretty nice load/match to the transceiver.

I haven't had the opportunity yet to do a road test with this antenna and will do so within the next week or so while commuting. I hear signals and Hams jabbering away on all active bands and was especially taken back by the strength of the RTTY and PSK31 signals I hear - they were STRONG.....gee, wonder if I can use a laptop in my car now to work digital modes?!

I have a couple of mechanical tweaks to do before the antenna is a 'done deal' and I may experiment with a physically (and electrically) shorter radiating antenna to see what the performance does. VSWR plots of those will appear within the next week or so. I will get all this done before this antenna is really road-worthy - - but I'm anxious to get rolling with this antenna for my long boring commutes each day so I can work HF in the car without being strapped to a single band (like my Perth Plus is).

Final pictures of the completed antenna will begin appearing on my website for all phases of this project (from piece parts to finished goods). Be looking for those in the next 36 to 48 hours. Another message on this blog will follow with the links to those pictures.

But the bottom line is: This design idea works! I'm now planning to sell my Perth Plus on QTH.com - - it's history. This new Multiband Vertical HF Mobile Antenna is taking over for the long haul for all my HF mobile communications.

As usual - Stay tuned for more trials and tests with this new antenna design....

73's de Marty, KN0CK

[marty_wittrock]

All the pictures of this Multiband HF Vertical Mobile Antenna have been uploaded to my website at the following link:

http://www.martywittrock.com/MBHFVMA/

All you have to do is click the link above and wait for the pictures to load. You'll be able to see all the initial construction of the antenna pieces and the final result mounted on my Honda.

I did a check-in to the Iowa 75m net this evening and was heard by the net control station in central Iowa - so the antenna is functioning on groundwave. Based on the default power output of the FT-817, I need to re-tune it to the higher power setting that is possible with this rig and try more contacts with the 75m whip. I also tuned the 75m whip to the upper end of the 75m band (with good VSWR over 3.7 to 3.999 MHz - less than 1.5:1) and heard MANY stations on the air.. No change in VSWR over the whole 80 - 10m bands - - is the same as it was this morning.

Also, I did a 'high speed' test with the 75m whip applied to this antenna and found that the magnetic base cannot sustain any more than 60 mph without tilting over (pulling away from the body of the Honda). The 75m whip is a 'friendly dog' and has A LOT of 'wagging' action when the wind blows against it. The fix for that is to apply a heavy plastic guy wire (i.e. black 'weed whacker' cord) from the trunk lid edge using a soft-plastic coated broad aluminum trunk lid 'hook' to the middle of the 75m whip to keep the wagging-action down to a minimum and keep the mag mount on the trunk lid. I'll apply that later. As well, I've purchased a 5/8 wave 24" CB whip from the local Truck Stop and did a cursory check of the VSWR 80 - 10m. I'll post the results later, but the highest VSWR observed was 2.5:1 at the high end the 80m band (3850 kHz to 4000 kHz). Everywhere else it was within 1.7 to 1.5:1 and from 12m and the entire 10m band the VSWR was a flat 1.0:1 match. One nice perk: it was possible to drive 75 mph with this smaller whip on the Honda without the mag mount coming loose (HI HI)!

More to follow on this antenna and my on-air results - Stay tuned......

73's de Marty, KN0CK

[marty_wittrock]

Since the last time I posted results of the Multiband Mobile Antenna (80 - 10m), I wrote that I was going to try some other antennas and provide the details of that. Well, I've tried a number of different antennas for VSWR and performance working with the matching transformer contained in the waterproof PVC housing that makes-up this unique antenna. I'll mention them in this posting, but ultimately I found that a simple 5/8 wave Francis black (not that any color matters here - orange looked funny on my car) fiberglass CB whip antenna was great for two things: workable VSWR over the operational bandwidth, and low wind resistance at high (80 mph as a test) speeds.

So with that covered, here's my observations:

At the get-go I tried a 80/75m Workman wirewound whip antenna. In my earlier message I posted the VSWR plot of that antenna and found that my 80/75m performance was a shade better than the antenna I'm running now. Seems that when you have a resonant antenna on top of the matching transformer (see my earlier posts on the transformer construction) will tend to have a lot more influence on the VSWR than if the antenna is not electrically correct. The VSWR where the antenna is resonant will be prominent and most of the energy is being transferred to the antenna. Everywhere else the matching transformer is picking up some of the energy, but the radiating element (antenna) isn't loafing - it's working, too. You'd never tell the difference in receive - it pulls in signals as well as any other antenna you can use. About the only bad thing that I can cite with the 80/75m antenna was that its wind resistance was SO great that at 60 mph it took the magnet mount right off the trunk of my car and the tip of the antenna dragged on the ground for about 100 ft while I was braking to a stop to correct the situation. All that said, performance of the 80/75m antenna was great. I was heard on the Iowa State-wide 80/75m net (at 3970 kHz), albeit with the QRP rig I was running barefoot, and could hear signals on all bands 160 - 10m with no issue. Even shortwave listening was fun with that antenna - - it pulled in A LOT of stations as well as my home setup.

About the same period of time, I purchased a 24" 5/8 wave CB whip as a compare/contrast to the 80/75m 7' long antenna's performance. Signals were noticeably lower with this antenna and the VSWR was much higher across the band. I didn't bother to take comprehensive data on this one because I knew I wouldn't be using it. It just didn't have the incredible receive performance of that 80/75m antenna. For what money I was out for buying it, the only advantage I had with that short antenna was that I could drive high speeds with it mounted on top of the matching transformer (80 mph as a test) - - but that reinforced the idea that a lighter, lower profile whip was needed to meet the two criteria: better performance and lower wind resistance.

I purchased a Workman 48" 40m antenna from a local radio shop at Walcott, Iowa along with a 36" Francis 5/8 wave fiberglass antenna as another compare contrast. I have to admit that I haven't taken data on the 40m antenna yet (that data is forthcoming) but what follows is the VSWR plot of that 5/8 wave fiberglass CB antenna:

3.500 MHz = 2.9:1
3.750 MHz = 2.8:1
3.970 MHz = 2.6:1
3.995 MHz = 2.6:1

7.000 MHz = 1.7:1
7.150 MHz = 1.8:1
7.200 MHz = 1.8:1
7.250 MHz = 1.8:1
7.300 MHz = 1.8:1

10.100 MHz = 1.7:1
10.125 MHz = 1.7:1
10.150 MHz = 1.7:1

14.000 MHz = 1.9:1
14.100 MHz = 1.8:1
14.200 MHz = 1.8:1
14.300 MHz = 1.8:1
14.350 MHz = 1.9:1

18.070 MHz = 1.6:1
18.100 MHz = 1.6:1
18.150 MHz = 1.5:1
18.165 MHz = 1.6:1

21.000 MHz = 1.1:1
21.100 MHz = 1.1:1
21.200 MHz = 1.1:1
21.300 MHz = 1.1:1
21.400 MHz = 1.1:1
21.450 MHz = 1.1:1

24.890 MHz = 1.1:1
24.890 MHz = 1.1:1
24.890 MHz = 1.1:1
24.890 MHz = 1.1:1

28.000 MHz = 1.2:1
28.250 MHz = 1.2:1
28.500 MHz = 1.3:1
28.750 MHz = 1.3:1
29.000 MHz = 1.3:1
29.250 MHz = 1.3:1
29.500 MHz = 1.3:1
29.700 MHz = 1.4:1

As you can see, the results are somewhat similar to the 80/75m antenna's VSWR plot. 80m tends to be the highest VSWR (average of 2.6:1 over the band) but the VSWR plots from 40m to 10m were very acceptable - - all less than 2.0:1. In fact, anything at or above 20m is downright workable - and even covers the WARC bands without any VSWR issues.

Performance of this 5/8 wave antenna was GREAT! I screwed it to the top of the matching transformer and started down the road. I was tuning across the 20m band and heard some strong signals around 14.230 MHz (if memory serves) coming from a QSO party station that was located in Grand Junction, Colorado. Here I was speeding down the road (at the test speed) on I-80 eastbound heading to Davenport, Iowa and made a call to the QSO party. They answered back and we exchanged signal reports. I had them 589 at that location (about 5 miles west of Davenport) and they had me at 579 with this mobile antenna. So the proof was there - this antenna was definitely a keeper!

About 2 weeks later I was going home by way of Highway 30 to my QTH using the same 5/8 wave antenna (this time at 60 mph) and was about 2 miles west of the Vinton turnoff at Highway 30 when I joined the Iowa 80/75m net. Usually we don't exchange signal reports on that net, but I was heard by the Net Control Station (if memory serves is near Des Moines) in fine shape - clear and readable, as I was motoring just south of Vinton. As well, I was heard better by stations in Brooklyn, Iowa that were near my QTH (no surprise there) as I got close to home. Again, I was assured that this was a workable antenna. Even the receive signal strength was on par with the 80/75m antenna.

Again, I still have a 40m antenna to check out for VSWR and overall receive and transmit performance, so there is one more posting before I finalize this.

So as usual, stay tuned for more details on that......

73's de Marty (KN0CK)

[KF5YFD]

I would like to see the results of the 40 m and any other antennas you have tried.