Antenna hacks - THE MANTOVA TURBO-8 EXPOSED
IMPROVEMENTS TO A BRAND NEW UNBOXED 5/8 WAVELENGTH MANTOVA TURBO. -by 1SFØ72
•PLEASE NOTICE: This document is about Mantova Turbo antennas but you can use information contained here to gather ideas and improve any other similar 5/8 antenna you already own.
I'm writing this in the hope it may come in handy if you're planning to refurbish your old 'Mantova turbo', or if you want your brand new one to last longer with maximum efficiency. First let me say I formerly had an old "Mantova-1, a 5/8 vertical antenna for the 27 megs with 4 radials that stood on top of my roof for more than 20 years with no big issues, aside a small loss of efficiency and a slight increase of VSWR during the last years of use.
I recently investigated the causes when I took the antenna down for maintenance and discovered that water rain, rust and aluminium oxidation were the main culprits; in addition, the fiberglass isolator holding up the radiating element was severely damaged after years of UV exposure and therefore I decided to buy a new antenna, swapping out the existing one for safety reasons.
I ordered a Mantova Turbo (8 radials) and a quarter of an hour after delivery it was lying on my lab table with its base completely disassembled in order to fix some well-known factory defects. In particular, I wanted to protect connectors and all critical parts from water infiltrations, then replace that small and inadequate piece of wire soldered to the center pin of the SO-239 connector that's badly attached to the other side, the feed point of the radiating element (whip).
A pic is worth a thousand words so let's have a look:
The true weakness of this antenna is its feed point; moisture and water rain enter here and go straight to the SO-239 connector. Moreover, the contact is highly subjected to oxidation because the copper wire from the connector is simply twisted around the steel bolt... The factory could have done far better!
(Above pic) The base of this antenna now is completely disassembled. I had to remove the steel bolt then I had to drill out the two small aluminium rivets in order to remove the big black plastic cover;
This antenna is DC grounded through the coil you can see in the above picture; if you needed to rebuild this coil you should wind 9 turns of 15 AWG (1.5mm) enameled copper wire. I tested the inductor and read about 4uH if my LC-meter is not stoned of course... (LOL)
I talked about water infiltrations, a common problem for all Mantova antennas. The solution? Get a cartridge of SMP Polymer adhesive and seal every gap, every unuseful opening/slit you may come across. Being this antenna DC grounded don't forget to insert that L-shaped brass strip first, in order to establish a fail-proof continuity between the base of the antenna (along with its radials) and the mast. Do not use a common silicone that's not UV resistant and never use too much sealant! Seal also the top of the plastic cap, just spread a very thin layer of sealant around the fiberglass isolator. You could also consider replacing that galvanized iron plate with a new identical plate you formerly made out of a robust aluminium sheet;
Now replace that ugly and thin wire soldered to the SO-239 connector with a more reliable 20 AWG (2mm) piece of wire;
Always do very good and strong joints! Cold joints are absolutely unreliable and will cause a lot of problems with your antenna. Furthermore, cold joints may break suddenly causing a failure that may lead your PA or transceiver to a serious damage.
Finally, protect the joint with self amalgamating tape then apply some sealant and spread it on the whole surface of the rear side of the connector. Let the sealant dry out completely and you'll be ready to reassemble all parts.
Next step, the radiating element feed point. Wrap some turns of PTFE tape (aka plumber's tape) around the plastic cap and prepare a ring cable eye as shown. I used this trick to protect the plastic beneath the tape from the heat of the soldering iron.
Do the soldering and again, do it good then clean the whole area and insert the bolt. After you tightened the nut use a multimeter to test continuity between the center pin of the SO-239 connector, the bolt and any other metal part of the block. If continuity is assured all the way then everything's fine and you're ready for the final steps.
Seal as shown, then put new rivets in their holes;
I had no rivets (damn, I hate 'em!) so instead of rivets I inserted a small and hard plastic stick long enough to reach the opposite hole (and sealed it on both sides);
Apply some sealant around the tube to fill the gap existing between the tube and the plate;
Now close the remaining gaps between metal and plastic and don't forget to spread a layer of sealant around the upper half of the connector plastic protection...
And finally, when the sealant dries out completely your antenna will be ready to be installed on the roof!
This is my new antenna facing the sky after 'the treatment'. It works very well with no VSWR issues and during my early tests some of you gave me superb reports. In the next years I'm expecting it to work like the first day I put it on the roof. If you're planning to refurbish/restore your antenna don't forget to protect all whip junctions as well as the PL-259 connector with self amalgamating tape. RF-chokes are used to stop common mode currents from flowing through the braid of the coax on TX but even reception will be improved. If you made an RF-choke bear in mind that it should be band-specific and not randomly wound! Even on a multi-band antenna a good choke would be realistically effective in small or large band segments near the frequency of interest but *NOT* in the entire HF spectrum. That's the reason good chokes can't be 'multi-band' to be truly effective, and that's why they require accurate tests and measurements prior to use. That's it, I have nothing else to say at the moment and therefore I'm gonna stop it here, just feel free to experiment. I did it, so you can do it too! Of course remember, you follow this guide at your own risk. I don't know your abilities and skills and will take no responsibilities on what you decide to do with your antennas. Always be on guard when working on top of tall buildings. Never forget WHERE you are, WHAT you're doing and above all, never, ever do anything stupid such as walking backwards while taking pictures, THE ROOF OF A BUILDING, EVEN WHEN IT'S FLAT, *IS ALWAYS A DANGEROUS PLACE TO BE* (YOLO). If you're not confident with works at a height don't be afraid to ask for help or search for qualified assistance. So again, stay away from troubles and above all... have fun.