A certain formula for antenna system Q

A correspondent questioned the writings of an online expert who opined whilst discussing loaded monopole antennas:

… there is a formula circulating the Internet which states that antenna Q is equal to 360 times the frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz. One has to assume they mean antenna system Q, but that’s not a given. While this formula might give you a comparison between antenna A and antenna B (all else being equal), the actual Q of the antenna (system or otherwise) requires a textbook-full of formulas, and a lot more information than just the 2:1 bandwidth! Fact is, this formula is no more specific than the number of DX contacts a specific antenna garnered.

The formula given is:

Q=360*fc/B(VSWR=2) where fc is the centre frequency.
Continue reading A certain formula for antenna system Q

Tuning combiner lines

A common method of combining two 50Ω antennas to a single 50Ω feed is using a quarter wave transformer using 75Ω line from the common feed point to each antenna.

A recent posting to one of the ham fora raises the posters problems with making this really simple feed system work.

Screenshot - 25_08_16 , 07_25_16

Above is his measured input characteristic with good 50Ω loads on each leg. Reading a hundred posts, it seems that he attributes this to legs of 0.167m length of RG11. The problem is that RG11 as most of us know it has a solid PE dielectric giving it a vf=0.66 and that 0.167m is 63° at 207MHz… so why the response above. Continue reading Tuning combiner lines

Treatment of the -ve DC return path for transceivers in mobile installations

A correspondent wrote with questions on the -ve return connection in a mobile installation of a typical ham transceiver. He was confused by the advice on an online expert who opined…

If instead, you decide to connect the negative lead to the nearest chassis ground point (seat support, trunk brace, etc.), there will be a difference in resistance between any of these points and the battery’s chassis ground. A differential of three to five ohms is not uncommon. Whether this causes a ground loop to occur is moot, the resulting voltage drop under load is not.

A resistance of 3-5Ω from any part of a metal car body to the terminal clamped to the battery -ve terminal is way above anything I have observed, and would seem to be sign of a fault rather than not uncommon. Continue reading Treatment of the -ve DC return path for transceivers in mobile installations

Exploiting your antenna analyser #23

Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser.

Screenshot - 03_08_16 , 14_12_56

Above is a sweep of an A/B changeover relay intended for HF application at up to 100W and lowish VSWR. The sweep is actually from 1 to 61MHz to be confident that there is not poor behaviour just outside of the HF range that might present on another implementation of the same design. Continue reading Exploiting your antenna analyser #23

End fed half wave – NEC models for 20m

Introduction

End Fed Half Wave antennas are again very fashionable with hams, accompanied by extraordinary claims and somewhat sparse understanding (the way of modern ham radio).

To add some light I have created a set of NEC 4.2 models of a half wave antenna on 20m to give some insight into the behaviour of a bottom fed vertical half wave over real ground.

This analysis does not consider harmonic operation, antennas are a half wave at 14.2MHz.

Four models are used:

  1. 20mHW-VEP – bottom fed vertical above perfect ground;
  2. 20mHW-VEA – bottom fed vertical above real ground;
  3. 20mHW-VCA – centre fed vertical above real ground (ie ground independent feed);
  4. 20mHW-HCA – centre fed horizontal at 5m height above ground;

NEC 4.2 model description:

  • 14.2MHz;
  • no conductor loss;
  • real ground assumed to have conductivity=0.005S, εr=13, of course results are dependent on these values;
  • conductors are ~10m long, 20mm diameter;
  • bottom fed vertical half wave uses a 10m x 20mm vertical driven ground electrode;
  • centre fed vertical is raised 200mm above ground;
  • feed line and feed line common mode current are excluded;
  • the centre of all antennas is ~5m above ground (real or perfect).

clip-230

Above are the patterns from the models for discussion. Continue reading End fed half wave – NEC models for 20m

InsertionVSWR of Grebenkemper’s Tandem Match

The findings at InsertionVSWR of Revex W560 on HF and the suggestion that the low frequency problem is characteristic of poorly designed Sontheimer couplers (Sontheimer, C & Frederick 1966).

These couplers were popularised by (Grebenkemper 1987)  in his Tandem Match – An Accurate Directional Wattmeter and have appeared in ARRL handbooks over the decades, and may have inspired the many commercial implementations of the coupler.

Grebenkemper claims his meter is ‘good’ down to 1.8MHz, but does not clearly claim any particular InsertionVSWR. There is limited value in an instrument that can measure down to 1.05 when it causes significantly higher VSWR itself.

Screenshot - 24_07_16 , 15_31_41

Lets drill down on Gebenkember’s article, specifically the coupler design.
Continue reading InsertionVSWR of Grebenkemper’s Tandem Match

KG-UV920P repair / modification for driver FET failure

The KG-UV920P is infamous for failure of the driver FET, they run excessively hot and clearly outside of safe operating limits.

I repaired one for a friend some years ago, and the dealings with Wouxun were enlightening. If I had little confidence in them before that experience, after it I would not give consideration to purchase of any Wouxun radio.

My repair / modification notes have been copied literally from the old VK1OD.net webside, and may contain stale links etc, but if it is of use to hams with a broken radio, see KG-UV920P – a repair / support story.

I have seen lots of articles on this problem over the years since, including ones that try to add a heatsink on top of the driver FET. The driver FET is meant to lose its heat through the bottom metal pad, and heatsinking the plastic encapsulation will not be very effective. Bottom line is to reduce the operating voltage on the driver (as per the factory advice), and keep the radio cool.

Don’t operate the radio sitting on the desk or the like, the bottom is the heatsink.

Wouxun are not alone in manufacturing radios that run red hot, see my notes on supplementary cooling for an IC-220H: http://owenduffy.net/blog/?s=IC2200H+cooling.

 

Fan controlled by humidity sensor

I have a problem with machine tools getting condensation on them when conditions in the shed read dew point.

A possible solution being explored is to circulate air with a fan, possible inducting outside air, when humidity approaches condensing conditions.

A quick search reveals the HDS10 resistive humidity sensor for a dollar or so on eBay.

Above is the HDS10 humidity sensor.

Most low cost humidity sensors use a humidity dependent capacitive element, the HDS10 is different in being a humidity dependent capacitive element and is therefore simpler to use with microcontrollers with ADC input.

HDS10-02

The above graph is from the datasheet. It is intended primarily for sensing high humidity (dew point, condensing conditions) which suits this application.
Continue reading Fan controlled by humidity sensor