When sorting out some old file I’ve found an old video and the story linked to it.
Some two years ago I’d got the plan to decorate my roof gear with a Hygain AV-620. I had heart a lot of praise about this antenna and I could buy a used one in good condition and for not too much money. But before this huge thing was mounted on the mast I wanted to be sure it had been tuned correctly. So I decided to assemble the Hygain safely on the ground. After a circus act on a ladder I’ve decided to climb on the veranda to put the antenna on a stable tripod (the type for loudspeakers) to do some testing.
A coax cable of a sufficient length was connected to the antenna, and at a respectable distance the antenna analyzer (MFJ-269) was connected to the cable. I was surprised to see the needles of the analyzer jumping up and down. Although there was nothing and nobody close to the antenna. A broken coax? But in that case the analyzer should show an infinite SWR? Just to be sure I decided to take another cable, but with the same result.
Slowly I got the feeling there was something wrong at the base of the antenna. Again the circus act on the (too) short ladder. And after checking all screws, the result remained the same. At last I took the antenna apart again to rethink everything.
The next weekend the whole setup was built up again. Again I lowered the antenna from the veranda on to the tripod. To be on the safe side I put the tripod a bit further from my veranda so the Hygain was pointing freely into the air. But unfortunately the experience of the previous week repeated itself. Wild waving needles and strange readings on the LCD display. Broken analyzer? To test the analyzer I’ve connected it to a VHF antenna, and everything turned out to be okay. Again the antenna was taken apart again, again I didn’t have the slightest idea what went wrong.
During that week I was looking out of the window at the other side of my house. And I was facing the answer! The broadcast transmitter ‘Hulsberg’ is at a distance of less than 1 kilometer. In those times it was still throwing Radio 538 into the air with more than 20 kW! No idea what the strength of its EM field is at that distance, but for sure it’s more than a portable radio for a HAM. And the power the MFJ uses to measure SWR was completely overwhelmed by the AM broadcast transmitter. No wonder the needles where bouncing in a certain rhythm… on the rhythm of Radio 538!
Totdat ik in de loop van diezelfde week aan de andere kant van het huis uit het raam keek. Ik keek het antwoord recht in de ogen! Op minder dan 1 kilometer staat de omroepzender ‘Hulsberg’, die op dat moment nog met meer dan 20 kW radio 538 de ether in slingerde! Geen idee over welke veldsterkten we het dan hebben, maar ook op die afstand is het beslist meer dan een amateur portofoontje! En het vermogen dat de MFJ voor de meting gebruikte, werd natuurlijk compleet onder de voet gelopen door de AM-omroepzender. Geen wonder dat de naaldjes van ritmisch op en neer hupten… Op de maat van radio 538!
At last the Hygain was assembled again to be tested with a QRP transceiver and an SWR meter. Then I found out the antenna had been assembled correctly and could be mounted on the mast. Later on this Hygain brought much more sorrow. But that can be found elsewhere on the site.