HF radio transmitter, open feedline, and neon lamps ? - Page 1
Forum section: electronics / rf-microwave

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 03, 2024, 11:31:22 pm
content: I hold a US/FCC granted General class license. As such, when transmitting on the HF bands I'm using one of two doublets, each with a 530Ω open feedline.Today, while trying to organize my parts stash, I found a very old box of Chicago Miniature Lamps, type C2A (aka NE-2H). Since neon lamps are similar to a gas discharge tube (but visual), can I use one to assist in tuning the match for my open feedline ? The antenna tuner in use is a MFJ-949E, which has the dual indicators for forward/reverse power. Searching the internet, I found one mention of a homebrew tuner that had a neon indicator wired across the feedline connectors. It wasn't clear if the indicator lit when the match was good, or when there was reflected power. I know the tuner I'm using works as desired, but I'm curious about the use of a neon indicator.One doublet is a 20m full wave (about 15 feet off the ground), and the other is a 40m full wave NVIS about 8 inches off the ground. Both perform as desired. The longer one matches down to 160m, while the short one will match up to 6m (but the antenna tuner isn't designed for that band).

author: mag_therm
author_rank: Frequent Contributor
timestamp: March 04, 2024, 01:57:45 am
content: Interesting as I also have a 20m EFHW fed with a 1/4 wave twin line transformer.There is this paper from 2016.https://etd.auburn.edu/bitstream/handle/10415/5207/Chen_Thesis%205.13.2016.pdf?sequence=2Section 2.3 and Fig 2.6 shows the variation of breakdown between electrodes (in this case air) is a non linear function of frequency in therange of HF ham bands. That might be a problem for using NE2 brightness for matching over different bands.I have a few old, surplus NE2 here. They had inconsistent breakdown and some would not strike near 90V.I read in old GE data that NE2 were "burned in" after manufacture or long storage.I made a full wave 120V unfiltered supply on variac/isolation transformer, and ran a batch each with a series resistor for a few days.It did improve most of them . Your old box may benefit from that too.For matching here I use nanoVNA. The small screen was no good for me , but after I ran it via nanoVNAsaver on desktop(or on a laptop when outside) it has been a great success.For operation I have an el cheapo 'scope set up permanently as Spectrum Anayser of the output of the home brew linear Amplifier via a TeeThat tells in real time if the antenna changed. For example this winter , snow on the crossarm of the feeder,and the ice on the antenna in that cold spell melted during ft8 and then ran down the catenary and refroze,putting the antenna way out of tune for a few days.Regards

author: LaserSteve
author_rank: Super Contributor
timestamp: March 04, 2024, 08:41:19 am
content: Is your feeder insulated?If insulated, mount lamp on long plastic stick for your safety.Spread leads of lamp to make capacitive coupling.In theory, if you have enough TX power and high enough Q, lamp may light as you slide it along the ladder line. in theory you can see the standing wave. Remember, in the vacuum tube final amplifier days there already was a HV field to help ionize the gas.Basically forms a Letcher line.Other variations may include coupling to lamp with a high q parasitic tuned LC circuit. Not all lamps used for this were neon.A diode detector and meter may work as well.Older ARRL handbooks may have better detailsAbout halfway through this excellent training video is an extreme example with a four watt lamp;https://youtu.be/CLzWXJfpleQ?si=aLBP5DJcNfh3F8moSteve

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 04, 2024, 11:19:32 am
content: I am still reading about how this might work. My best understanding is that when the impedances are not matched, the voltages on the two wires will be out of phase with one another. The objective here is to get the voltage phases, at the antenna feedpoint, to be exactly 180º on opposite sides of the antenna. So that one side will be maximum positive voltage, while the other is maximum negative voltage. When the antenna is not matched correctly, those peaks will be out of phase with one another and reflected current will happen. This is an admittedly simple interpretation of the theory at work.I'm also aware that I will need a ballast resistor (so that the neon lamp strike voltage will be sufficiently high, with relation to the delta voltage between the two wires). The value of the ballast resistor may change if my RF power level changes (as that would affect the RF voltage on the feedline wires). Normally I'm running 25 watts.